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RESEARCHES 


UPON 


THE  ATOMIC  WEIGHTS 


OF 


CADMIUM,     MANGANESE,     BROMINE, 

LEAD,    ARSENIC,    IODINE,    SILVER 

CHROMIUM,  AND  PHOSPHORUS 


BY 


GREGORY    PAUL    BAXTER 


IN    COLLABORATION    WITH 


M.    A.    HINES,    H.    L.    FREVERT,    J.    HUNT    WILSON,    F.    B.    COFFIN, 

G.  S.  TILLEY,  EDWARD  MUELLER,  R.  H.  JESSE,  Jr., 

AND   GRINNELL  JONES 


<"HHSTJMUT  HiLi^  l^iASa. 
WASHINGTON,  D.  C. 

PUBLISHED    BY    THE    CARNEGIE    INSTITUTION    OF   WASHINGTON 

I9IO 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
PUBLICATION  No.  135 


CONTRIBUTIONS  FROM  THE  CHEMICAL  LABORATORY 
OF  HARVARD  COLLEGE 


QDy 


63 


'tK3 


2132-34 

THE    UNIVERSITY    PRESS,   CAMBRIDGE,    U.S.A. 


PREFACE. 

This  collection  of  papers  upon  the  atomic  weights  of  certain  common  ele- 
ments embodies  the  results  of  researches  of  which  the  experimental  work  has 
been  carried  on  in  the  Chemical  Laboratory  of  Harvard  College  during  the 
past  six  years.  All  of  the  papers  have  already  been  published  separately  both 
in  American  and  in  German  periodicals,  and  references  to  the  places  of  publi- 
cation are  given  at  the  beginning  of  each  article. 

In  reprinting  the  papers  in  the  present  form  the  only  changes  of  importance 
which  have  been  made  are  due  to  more  exact  knowledge  of  the  fundamental 
atomic  weights  upon  which  the  calculations  depend.  Many  recent  investiga- 
tions, especially  that  upon  the  analysis  of  lithium  chloride  and  perchlorate  by 
Richards  and  Willard,^  have  shown  that  the  atomic  weight  of  silver,  referred  to 
oxygen  16.000,  is  certainly  as  low  as  107.880,  and  possibly  as  low  as  107.870. 
Since  the  International  Committee  upon  Atomic  Weights  at  the  date  of  writ- 
ing have  chosen  the  higher  of  these  values,  the  calculations  have  been  based 
upon  the  value  107.880  for  silver,  the  atomic  weights  of  chlorine  and  bromine 
being  assumed  to  be  35.457^  and  79.916  ^  respectively.  The  effect  of  a  change 
from  107.880  to  107.870  in  the  atomic  weight  of  silver  is,  however,  plainly 
indicated  in  each  instance. 

In  the  case  of  cadmium  the  subject-matter  of  two  papers  has  been  rear- 
ranged in  a  manner  differing  considerably  from  that  of  the  original  publica- 
tion. In  the  case  of  iodine  the  subject-matter  of  two  papers  has  been  com- 
bined in  one.  In  all  other  cases  the  presentation  is  essentially  that  of  the 
original  publication. 

Generous  grants  from  the  Carnegie  Institution  of  Washington  have  been 
of  the  greatest  assistance  in  the  progress  of  this  work,  while  grants  from  the 
Cyrus  M.  Warren  Fund  for  Research  in  Harvard  University  have  materially 
aided  all  the  investigations.  G.  P.  Baxter. 

*  Publications  of  the  Carnegie  Institution,  No.  125  (1910);  Jour.  Amer.  Chem.  Soc,  32,  4. 

*  Puhlications  of  the  Carnegie  Institution,  No.  28  (1905);  Jour.  Amer.  Chem.  Soc,  27, 
459;  Zeit.  anorg.  Chem.,  47,  56. 

8  Proc.  Amer.  Acad.,  42,  201  (1906);  Jour.  Amer.  Chem.  Soc,  28,  1322;  Zeit.  anorg. 
Chem..  Soc,  389.   (See  page  49.) 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 

Boston  Library  Consortium  IVIember  Libraries 


http://www.archive.org/details/researchesuponatOObaxt 


CONTENTS. 

Page 

Preface '.  .  iii 

I.  A  Revision  of  the  Atomic  Weight  of  Cadmium;  The  Analysis  of  Cadmium 

Chloride.    By  G.  P.  Baxter  and  M.  A.  Hines. 

Introduction 3 

Purification  of  Materials 4 

Preparation  of  Cadmium  Chloride  for  the  Preliminary  Analyses 7 

Method  of  Analysis lo 

Preliminary  Series  of  Results 13 

Action  of  Hydrochloric-Acid  Gas  upon  Phosphorus  Pentoxide 14 

Preparation  and  Drying  of  Cadmium  Chloride  for  the  Final  Analyses 15 

Final  Series  of  Results 16 

II.  A  Revision  of  the  Atomic  Weight  of  Cadmium;  The  Analysis  of  Cadmium 

Bromide.    By  G.  P.  Baxter,  M.  A.  Hines,  and  H.  L,  Frevert. 

Purification  of  Materials 21 

Drying  of  Cadmium  Bromide  for  Analysis 22 

Method  of  Analysis 24 

Results  and  Discussion 26 

III.  A  Revision  of  the  Atomic  Weight  of  Manganese;  The  Analyses  of  Man- 

ganous  Bromide  and  Chloride.    By  G.  P.  Baxter  and  M.  A.  Hines. 

Introduction  .    ." 33 

Analysis  of  Manganous  Bromide 34 

Purification  of  Materials 34 

Drying  of  Manganous  Bromide     37 

Method  of  Analysis 38 

Density  of  Manganous  Bromide 40 

Results 42 

Analysis  of  Manganous  Chloride 44 

Piurification  of  Materials 44 

Drying  of  Manganous  Chloride     44 

Method  of  Analysis 45 

Density  of  Manganous  Chloride 46 

Results  and  Discussion 46 

IV.  A  Revision  of  the  Atomic  Weight  of  Bromine;  The  Synthesis  of  Silver 

Bromide  and  the  Ratio  of  Silver  Bromide  to  Silver  Chloride.    By  G.  P. 
Baxter. 

Introduction 51 

Purification  of  Materials 54 

Synthesis  of  Silver  Bromide 57 

Results  _ 58 

Conversion  of  Silver  Bromide  into  Silver  Chloride 59 

Results  and  Discussion 60 

V.  A  Revision  of  the  Atomic  Weight  of  Lead;  The  Analysis  of  Lead  Chloride. 

By  G.  p.  Baxter  and  J.  H.  Wilson. 

Introduction 65 

Purification  of  Materials 67 

Drying  of  Lead  Chloride  and  Method  of  Analysis      68 

Results  and  Discussion 69 

v 


VI  CONTENTS 


VI.    A  Revision  of  the  Atomic  Weight  of  Arsenic;   The  Analysis  of  Silver 
Arsenate.    By  G.  P.  B.\xter  and  F.  B.  Coffin. 

Introduction 73 

Preparation  of  Trisilver  Arsenate 74 

Purification  of  Other  Materials 76 

JNIethods  of  Analysis 77 

Insoluble  Residue      81 

Determination  of  Moisture  in  Dried  Silver  Arsenate 82 

Specific  Gravity  of  Silver  Arsenate 84 

Results  and  Discussion 85 


VII.  A  Revision  of  the  Atomic  Weight  of  Iodine;  The  Synthesis  of  Silver 
Iodide  and  the  Ratio  of  Silver  Iodide  to  Silver  Bromide  and  Silver 
Chloride.    By  G.  P.  Baxter. 

Introduction 91 

Ratio  of  Silver  to  Silver  Iodide 92 

Purification  of  Materials 92 

Method  of  Synthesis 94 

Specific  Gravity  of  Silver  Iodide 96 

Results 97 

Ratio  of  Silver  to  Iodine 99 

Results loi 

Ratio  of  Silver  Iodide  to  Silver  Chloride 102 

Results 104 

Ratio  of  Silver  Iodide  to  Silver  Bromide 105 

Results 106 

Ratio  of  Iodine  to  Silver  and  Silver  Iodide 107 

Discussion  of  Results no 

Ratio  of  Silver  Bromide  to  Silver  Chloride in 

Historical  Discussion 112 

Summary 114 


VIII.  A  Revision  of  the  Atomic  Weights  of  Iodine  and  Silver;  The  Analysis 
OF  Iodine  Pentoxide.    By  G.  P.  Baxter  and  G.  S.  Tilley. 

Introduction 117 

Purification  of  Materials  for  the  First  Series  of  Analyses 118 

Conversion  of  Iodic  Acid  into  Iodine  Pentoxide 121 

Determination  of  Iodine  in  Iodine  Pentoxide 123 

Determination  of  Moisture  in  Iodine  Pentoxide 126 

Specific  Gravity  of  Iodine  Pentoxide 129 

Adsorption  of  Air  by  Iodine  Pentoxide 130 

Purification  of  Iodic  Acid  and  Silver  for  the  Second  Series  of  Analyses 132 

Method  of  Analyses 133 

Discussion  of  Results 135 


IX.    A  Revision  of  the  Atomic  Weight  of  Chromium;  The  Analysis  of  Silver 
Chromate.    By  G.  P.  Baxter,  Ed.  Mueller,  and  M.  A.  Hines. 

Introduction 139 

Purification  of  Materials 141 

Preparation  of  Silver  Chromate 142 

Drying  of  Silver  Chromate      144 

Determination  of  Silver  in  Silver  Chromate 145 

Determination  of  Moisture  in  Dried  Silver  Chromate 147 

Specific  Gravity  of  Silver  Chromate      149 

Discussion  of  Results 151 


CONTENTS  Vll 


X.  A  Revision  of  the  Atomic  Weight  of  Chromium;    The  Analysis  of  Silver 

Bichromate.    By  G.  P.  Baxter  and  R.  H.  Jesse,  Jr. 

Introduction 155 

Purification  of  Materials     156 

Preparation  of  Silver  Dichromate 157 

Drj'ing  of  Silver  Dichromate 158 

Determination  of  Silver  in  Silver  Dichromate ,   .  158 

Determination  of  Moisture  and  Nitric  Acid  in  Dried  Silver  Dichromate 159 

Specific  Gravity  of  Silver  Dichromate 161 

Discussion  of  Results 163 

XI.  The  Revision  of  the  Atomic  Weight  of  Phosphorus;  The  Analysis  of  Silver 

Phosphate.    By  G.  P.  Baxter  and  Grinnell  Tones. 

Introduction 167 

Purification  of  Materials      169 

Preparation  of  Trisilver  Phosphate 171 

Drying  of  Silver  Phosphate 176 

Determination  of  Silver  in  Silver  Phosphate 177 

Insoluble  Residue     178 

Determination  of  Moisture  in  Dried  Silver  Phosphate 181 

Specific  Gravity  of  Silver  Phosphate 182 

Adsorption  of  Air  by  Silver  Phosphate 183 

Ratio  of  Silver  Bromide  to  Silver  Phosphate 184 

Discussion  of  Results 184 

Summary 185 


I. 

A    REVISION   OF   THE   ATOMIC   WEIGHT   OF 

CADMIUM. 

THE  ANALYSIS  OF  CADMIUM  CHLORIDE. 


By  Gregory  Paul  Baxter  and  Murray  Arnold  Hikes. 


Journal  of  the  American  Chemical  Society,  27,  222  (1905) ;  28,  770  {1906). 
Zeitschrift  fur  anorganische  Chemie,  44,  158  (1905) ;  49.  A^S  (1906). 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF  CADMIUM. 
THE   ANALYSIS  OF  CADMIUM   CHLORIDE. 


INTRODUCTION. 


From  the  following  list  of  investigations  upon  the  atomic  weight  of  cad- 
mium it  can  be  seen  that  this  subject  has  attracted  considerable  attention, 
especially  in  recent  years.^ 


Date. 

Investigator. 

Reference. 

Ratio  determined. 

Result. 

i8i8 

Stromeyer    .... 

Schweigger's  Jour.,  22, 

Cd:  CdO 

iii-S 

366 

Cd:  CdS 
Cd:  CI2 
Cd:l2 

II3-8 
112.8 
III. 7 

i8S7 

von  Hauer  .... 

J.  pr.  Chem.,  72,  350 

CdS04:  CdS 

111.9 

i860 

Lenssen 

J.  pr.  Chem.,        281 

CdCzOi:  CdO 

X12.0 

i860 

Dumas 

Ann.  Chem.  Pharm.,  113, 

CdCl2:  2Ag 

112. 14 

1881 

Huntington      .    .    . 

27 
Proc.  Amer.  Acad.,  17,  28 

CdBrz:  2Ag 
CdBr2:  2AgBr 

112.18 
112. 17 

1890 

Partridge     .... 

Amer.  Jour.  Sci.,  (3)  40, 

CdCsO*:  CdO 

111.80 

377 

CdS04:CdS 
CdCaOi:  CdS 

111.73 
111.67 

1891 

Morse  and  Jones 

Amer.  Chem.  Jour.,  14, 

Cd:  CdO 

112.07 

261 

CdCjOi:  CdO 

112.02 

1892 

Lorimer  and  Smith 

Zeit.  anorg.  Chem.,  i,  364 

CdO:  Cd 

112.04 

189s 

Bucher 

Doctoral  Dissertation, 

CdC204:  CdO 

111.88 

Baltimore,  Md. 

CdCzO^:  CdS 
CdCU:  2AgCl 
CdBra:  2AgBr 
Cd:  CdS04 
Cd:  CdO  (porcelain) 
Cd:  CdO  (platinum) 

112. 12 
112.31 

112.33 
112.3s 
112.08 
111.89 

1896 

Hardin 

Jour.  Amer.  Chem.  Soc, 

CdCl2:Cd 

112.03 

18,  1016 

CdBra:  Cd 
Cd:Ag 

112.02 
111.94 

1898 

Morse  and  Arbuckle 

Amer.  Chem.  Jour.,  20, 
536 

Cd:  CdO 

112.38 

The  relative  value  of  many  of  these  determinations  has  already  been  several 
times  discussed,^  and  since  it  is  invariably  a  difficult  matter  intelligently  to 

*  The  greater  portion  of  this  list  is  to  be  found  in  Clarke's  "  Recalculation  of  the 
Atomic  Weights,"  Smithsonian  Misc.  Coll.,  1910. 

The  results  have  been  calculated  with  the  use  of  the  following  atomic  weights: 
0  =  16.00;  C  =  i2.oo;  8=32.07;  Cl  =  35.46;  Br  =  79.92;  Ag  =  io7.88;  1=126.92. 

^  Clarke,  Partridge,  Morse  and  Jones:  Loc.  cit.;  Richards:  Amer.  Chem.  Jour.,  20,  547 
(1898). 

3 


4  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

criticize  experimental  work  without  an  actual  repetition  of  the  experiments, 
for  frequently  some  constant  source  of  error  is  so  securely  hidden  that  it  may 
be  detected  only  by  the  most  careful  investigation,  no  attempt  at  criticism 
is  made  here. 

Nevertheless,  it  is  worth  while  calling  especial  attention  to  the  careful  re- 
searches of  Morse  and  Jones  and  Morse  and  Arbuckle  upon  the  ratio  Cd  :  CdO, 
which  yielded  the  value  1 1 2.38,  and  that  of  Bucher  upon  the  ratios  CdCU  :  2  AgCl 
and  CdBr2  :  '2AgBr  which  yielded  the  values  112.31  and  112.33.  Our  experi- 
ments indicate  that  the  real  value  for  the  constant  in  question  is  sUghtly 
higher  than  any  of  these  results. 

In  a  recent  determination  of  the  specific  gravity  of  cadmium  chloride/ 
anhydrous  cadmium  chloride  was  prepared  by  ignition  of  a  double  chloride 
of  cadmium  and  ammonium  in  a  current  of  hydrochloric-acid  gas,  in  a  state 
of  so  great  purity  that  it  was  considered  worth  while  to  make  use  of  the  salt 
for  a  determination  of  the  atomic  weight  of  cadmiimi. 

PURIFICATION   OF   MATERIALS. 
CADMIUM    CHLORIDE. 

The  general  method  of  purification  of  the  cadmium  material  was  that  of 
fractionally  precipitating  cadmium  sulphide.  One  kilogram  of  metalHc  cad- 
mium was  dissolved  in  aqua  regia,  and  the  solution,  after  being  boiled  to  expel 
chlorine  and  oxides  of  nitrogen,  was  filtered,  and  diluted  to  about  4  liters. 
The  solution  contained  traces  of  lead,  copper,  thallimn,  nickel,  iron,  and  zinc. 
When  a  current  of  hydrogen  sulphide  was  passed  through  the  solution,  the 
first  small  fraction  of  cadmium  sulphide  which  was  precipitated  was  dark- 
colored,  nearly  black,  owing  to  the  presence  of  lead,  copper,  and  thallium. 
This  fraction  was  removed  by  filtration  and  rejected.  A  second  larger  fraction 
of  the  sulphide,  although  it  contained  no  appreciable  amount  of  the  latter 
metals,  also  was  discarded.  The  third  fraction  consisted  of  all  that  could  be 
precipitated  by  saturating  the  solution  with  hydrogen  sulphide.  However, 
this  did  not  contain  more  than  one-quarter  of  the  original  material,  for  the 
solution  was  very  strongly  acid,  owing  to  the  large  excess  of  acid  used  in  dis- 
solving the  metal  and  the  accumulation  of  the  acid  formed  during  the  precipi- 
tation. The  solution  was  separated  from  the  precipitate  by  decantation,  and 
was  then  diluted  to  16  liters.  Upon  saturating  this  solution  with  hydrogen 
sulphide  a  fourth  fraction  of  cadmium  sulphide  was  obtained,  and  a  second 
dilution  of  the  solution  made  possible  the  precipitation  of  nearly  all  the 
remainder  of  the  cadmium  in  still  a  fifth  fraction. 

The  third,  fourth,  and  fifth  fractions  of  the  sulphide  were  separately  washed 
until  free  from  chlorides.  As  the  electrolytes  were  eliminated,  the  cadmium 
sulphide  showed  a  tendency  to  pass  into  colloidal  condition,  which  necessitated 

*  Baxter  and  Hines:  Amer.  Chem.  Jour.,  31,  220  (1904). 


THE   ANALYSIS   OF   CADMIUM   CHLORIDE.  5 

long  standing  for  the  precipitate  to  settle  after  each  washing,  although  the  flasks 
which  contained  the  precipitates  were  kept  warm  by  being  placed  upon  a  steam 
radiator.  During  the  washing  the  original  fine  yellow  precipitate  was  gradu- 
ally converted  into  an  orange-red  crystalline  modification.  When,  as  was  usu- 
ally the  case,  both  forms  were  present  in  the  same  flask,  the  red  form  quickly 
settled  to  the  bottom  with  a  sharp  line  of  division  from  the  yellow  form.  Nearly 
all  the  yellow  form  was  changed  into  the  red  modification  upon  standing  about 
three  weeks. 

In  order  to  free  the  sulphide  from  included  and  occluded  impurities  each  frac- 
tion was  dissolved  and  reprecipitated.  The  red  form  of  the  sulphide  was  ap- 
parently nearly  insoluble  in  dilute  sulphuric  acid,  for  in  one  case  the  washed 
sulphide  was  boiled  with  the  acid  for  12  hours  without  any  appreciable  amount 
of  solution.  Finally,  hydrochloric  acid  was  used  to  dissolve  the  cadmium 
sulphide.  The  solution  of  each  fraction  was  diluted  to  8  liters  and  was  sat- 
urated with  hydrogen  sulphide.  Since  only  a  portion  of  the  cadmium  was 
precipitated  in  this  way,  owing  to  the  large  excess  of  acid,  the  acid  was  par- 
tially neutralized  with  ammonia.  This  resulted  in  the  precipitation  of  more 
cadmium  sulphide,  although  the  solution  still  contained  considerable  cadmium, 
for  cadmium  sulphide  is  soluble  to  a  marked  extent  in  an  acid  solution  of 
ammonium  chloride. 

The  sulphide  obtained  from  each  of  the  original  three  fractions,  both  before 
and  after  the  addition  of  ammonia,  was  combined  and  washed  until  free  from 
chlorides.  Each  fraction  was  dissolved  in  redistilled  nitric  acid,  then  enough 
redistilled  sulphuric  acid  to  convert  the  nitrate  into  sulphate  was  added,  and 
the  solutions  were  evaporated  and  the  residues  heated  until  all  volatile  acids 
were  expelled.  Finally  the  sulphate  was  recrystallized  three  times  from 
aqueous  solution. 

The  cadmium  sulphate  was  converted  into  cadmium  chloride  by  first  obtain- 
ing metallic  cadmimn  electrolytically.  A  saturated  solution  of  cadmium  sul- 
phate was  electrolyzed  with  about  one  ampere  current  per  square  decimeter 
in  a  platinum  dish,  which  served  as  the  cathode,  until  deposition  ceased.  After 
the  deposit  of  metal  had  been  thoroughly  washed  with  hot  water  until  free  from 
sulphate,  it  was  dissolved  in  hydrochloric  acid  which  had  been  distilled  with  the 
use  of  a  platinum  condenser. 

In  order  to  prepare  the  double  chloride  of  cadmiiun  and  ammonium  of  the 
formula  CdCl2NH4Cl,  the  calculated  amount  of  ammonium  chloride  was  added 
to  the  cadmium  chloride  and  the  solution  evaporated  to  crystallization.  This 
ammonium  chloride  was  synthesized  from  hydrochloric  acid  and  ammonia. 
The  hydrochloric  acid  had  been  distilled  in  platinum,  and  the  ammonia  had 
been  freed  from  amines  and  purified  as  follows :  Ammonium  chloride  was  boiled 
with  concentrated  nitric  acid  for  about  20  hours,  and  then  after  crystallization 
was  converted  into  ammonia  by  distillation  with  sodium  hydroxide.  The  solu- 
tion of  pure  ammonia  was  distilled  into  the  pure  hydrochloric  acid  in  a 


6  RESEARCHES    UPON    ATOMIC    WEIGHTS. 

platinum  dish,  and  the  solution  of  ammonium  chloride  was  evaporated  to 
crystallization. 

The  cadmium  ammonium  chloride  was  crystallized  in  a  platinum  dish,  eight 
times  in  the  case  of  the  first  fraction,  four  times  in  the  case  of  the  second  frac- 
tion. The  third  fraction  of  the  sulphide  was  not  converted  into  the  double 
chloride,  but  was  investigated  in  a  later  research  upon  cadmium  bromide  (see 
page  2i).  The  first  fraction  is  designated  as  Sample  I,  the  second  as 
Sample  II. 

A  third  specimen  used  in  the  analyses  was  a  portion  of  that  employed  in  the 
determination  of  the  specific  gravity  of  cadmium  chloride.^  This  is  designated 
as  Sample  A.  The  method  of  purification  of  Sample  A  was  almost  exactly 
identical  with  that  described  above,  except  that  the  original  material  was  pre- 
cipitated but  not  fractionated  with  hydrogen  sulphide. 

SILVER. 

In  the  preparation  of  pure  silver  essentially  the  same  method  was  employed 
as  in  other  atomic  weight  investigations  in  this  laboratory .^  In  this  case  the 
various  treatments  consisted  in  thrice  precipitating  the  silver  from  a  dilute 
solution  of  silver  nitrate  in  nitric  acid  by  a  large  excess  of  hydrochloric  acid, 
with  intermediate  reduction  of  the  silver  chloride  in  each  case  by  means  of 
invert  sugar  and  sodium  hydroxide.  The  sodium  hydroxide  for  the  third  re- 
duction was  freed  from  heavy  metals  by  electrolysis.  Both  the  silver  chloride 
and  the  metallic  silver  were  of  course  very  thoroughly  washed  by  decantation 
with  pure  water.  The  final  product  was  fused  on  charcoal  in  the  flame  of  a  clean 
blowpipe.  Next  the  buttons  were  converted  into  electrolytic  crystals  by  slow 
deposition  upon  a  pure  silver  cathode  from  a  concentrated,  nearly  neutral  solu- 
tion of  silver  nitrate,  the  anode  being  com-posed  of  the  pure  silver  buttons. 
After  thorough  washing  and  drying  the  crystals  were  fused  in  a  current  of  pure 
electrolytic  hydrogen  in  a  boat  of  pure  lime.^  The  lim.e  boat  was  made  by 
lining  an  unglazed  porcelain  boat  with  a  mixture  of  freshly  ignited  lim.e  and 
calcium  nitrate,  both  having  previously  been  carefully  freed  from  iron  and  other 
heavy  metals.  The  boat  was  thoroughly  ignited  before  use.*  During  the  fusion 
it  was  contained  in  a  large  Royal  Berlin  porcelain  tube,  the  ends  of  which  were 
closed  with  hollow  brass  stoppers  made  to  fit  tightly  by  means  of  narrow  r'ngs 
of  rubber,  the  stoppers  being  cooled  by  a  current  of  cold  water.^  Richards  and 
Wells,  in  a  recent  investigation  of  the  purity  of  silver  purified  by  different 

*  Loc.  cit. 

»  See  especially  Richards  and  Wells:  Ptib.  Car.  Inst.,  No.  28,  16  (1905);   Jour.  Amer. 
Chem.  Soc,  27,  472;  Zeit.  anorg.  Chetn.,  47,  70. 
'  Baxter:  Proc.  Amer.  Acad.,  39,  249  (1903);  Zeit.  anorg.  Chem.,  38,  237  (i904)- 

*  Richards:  Proc.  Amer.  Acad.,  30,  379  (1894);  Zeit.  anorg.  Chem.,  8,  262  (1895);  Rich- 
ards and  Parker:  Proc.  Amer.  Acad.,  32,  63  (1896);  Zeit.  anorg.  Chem.,  13,  89  (1897). 

'  Richards  and  Parker:  Loc.  cit. 


THE  ANALYSIS   OF   CADMIUM   CHLORIDE.  7 

methods,  have  found  that  silver  prepared  in  the  above  fashion  is  at  least  as 
pure  as  any/  Since  the  buttons  of  silver  obtained  from  the  fusion  in  hydrogen 
were  of  very  considerable  size,  they  were  cut  into  fragments  of  from  i  to  5  gm. 
by  means  of  a  clean  steel  chisel  and  anvil.  A  slight  surface  contamination  with 
iron  was  removed  by  etching  the  fragments  several  times  with  dilute  nitric 
acid,  until  the  acid  remained  free  from  iron,  and  drying  them  at  200°. 

Two  different  samples,  purified  in  the  same  way,  were  employed.  One  was 
prepared  especially  for  this  investigation  and  was  used  in  analyses  4,  5,  6,  and 
14.  The  other  was  a  portion  of  the  material  employed  in  an  investigation  upon 
the  atomic  weight  of  iodine  by  one  of  us  ^  (analyses  7,  8,  and  9).  Still  a  third 
specimen  of  silver,  used  in  analyses  13  and  15,  was  twice  deposited  electrolyt- 
ically  before  the  final  fusion  in  hydrogen. 

Water  was  purified  by  double  distillation  through  tin  condensers,  first  from 
alkaUne  permanganate  solution,  finally  with  a  trace  of  sulphuric  acid.  Con- 
nection between  the  flasks  and  the  condensers  was  made  by  constricting  the 
necks  of  the  flasks  to  fit  the  ends  of  the  condenser  tubes,  avoiding  thus  the  use 
of  rubber  and  cork.* 

Nitric  acid  was  twice  distilled  with  a  platinum  condenser,  the  first  third  being 
rejected  in  both  distillations.  The  product  of  the  first  distillation  contained 
only  the  merest  trace  of  chlorine. 

PREPARATION   OF   THE   CADMIUM    CHLORIDE   FOR   THE  PRELIMINARY 

ANALYSES. 

The  method  of  analysis  differed  little  from  that  used  in  the  analysis  of  other 
halogen  salts  in  atomic  weight  investigations  in  this  laboratory.  The  cadmium 
chloride  was  freed  from  ammonium  chloride  by  fusion.  Then,  after  solution  in 
water,  the  chlorine  content  was  found  either  gravimetrically  as  silver  chloride 
or  by  titration  against  weighed  amounts  of  silver. 

The  apparatus  used  for  the  expulsion  of  the  ammonium  chloride  from  the 
double  salt  was  similar  to  that  employed  for  a  Uke  purpose  by  Richards  and 
Parker*  in  their  analysis  of  magnesium  chloride.  Since  this  apparatus  was 
used  in  two  other  researches  described  in  this  collection,  a  detailed  description 
is  given  here.  Hydrochloric-acid  gas  was  generated  by  the  action  of  concen- 
trated sulphuric  acid  upon  concentrated  hydrochloric  acid  in  the  flask  A  (fig.  i), 
and,  after  bubbling  through  concentrated  hydrochloric-acid  solution  in  the 
wash  bottle  B,  it  was  dried  by  passing  through  four  towers  about  30  cm.  long 
and  4  cm.  in  diameter  filled  with  glass  beads  saturated  with  concentrated  sul- 
phuric acid,  C,  D,  E,  F.    In  the  first  series  of  experiments  the  hydrochloric-acid 

'  Loc.  cit. 

^  Baxter:  Proc.  Amer.  Acad.,  40,  419  (1904);  Jour.  Amer.  Chem.  Soc,  26,  1577;  Zeit. 
anorg.  Chem.,  43,  14  (1905).     (See  page  92.) 

3  Richards:  Proc.  Amer.  Acad., ^o,  2,^0  {iZg^);  Zeit.  anorg.  Chem. ,S,  261  (iSgs). 
*  Proc.  Amer.  Acad.,  32,  59  (1896):  Zeit.  anorg.  Chem.,  13,  85  (1897). 


8 


RESEARCHES   UPON  ATOMIC  WEIGHTS. 


gas  was  further  dried  by  passing  through  a  tube  containing  resublimed  phos- 
phorus pentoxide.  This  tube  is  not  shown  in  the  diagram  since  it  was  elim- 
inated in  the  final  series  of  experiments. 

Nitrogen  was  prepared  by  Wanklyn's  method  of  passing  air  through  concen- 
trated ammonia  solution  in  the  bottle  M  and  then  over  hot  copper  gauze  in  the 
hard  glass  tube  N.    The  excess  of  ammonia  was  removed  by  dilute  sulphuric 


=*=< 


Fig.   I.  —  Apparatus  for  the  fusion  of  chlorides  in  a  current  of  hydrochloric-acid  gas. 

acid  in  the  bottles  0  and  P,  and  the  nitrogen  was  purified  and  dried  by  means 
of  beads  saturated  with  silver-nitrate  solution  in  the  tower  Q,  solid  potassium 
hydroxide  in  the  tower  R,  concentrated  sulphuric  acid  in  the  towers  S,  T,  and 
U,  and  resublimed  phosphorus  pentoxide  in  the  tube  L. 

Air  was  purified  and  dried  by  reagents  similar  to  those  used  in  the  purifica- 
tion of  the  nitrogen,  in  the  towers  G,  H,  I,  J,  K. 

The  hydrochloric-acid  apparatus  was  constructed  wholly  of  glass  with  either 
ground  or  fused  joints,  while  the  nitrogen  and  air  purifjdng  trains  had  short 
rubber  connections  only  at  the  beginning.  Glass  gridirons  at  suitable  points 
gave  sufficient  flexibility  to  the  apparatus.  Ground  joints  were  made  tight 
by  means  of  either  concentrated  sulphuric  acid  or  with  syrupy  phosphoric 
acid. 

A  portion  of  the  cadmium  ammonium  chloride,  contained  in  a  weighed  plat- 
inum boat  in  the  hard-glass  tube  W,  was  heated  gradually  to  fusion  in  a  current 
of  hydrochloric-acid  gas  and  was  kept  fused  until  all  the  ammonimn  chloride 


THE   ANALYSIS   OF   CADMIUM   CHLORIDE.  9 

had  been  expelled.  After  the  salt  had  cooled,  the  hydrochloric  acid  was  dis- 
placed by  pure  dry  nitrogen  and  this  in  turn  by  dry  air.  Next,  by  means  of  a 
glass  rod,  the  boat  was  pushed  into  the  weighing-bottle  contained  in  the  soft- 
glass  tube  V,  and  the  stopper  was  inserted  without  opening  the  apparatus  or 
interrupting  the  current  of  dry  air  by  rotating  the  tube  V  slightly  so  as  to 
cause  the  stopper  to  roll  from  its  position  in  the  pocket  of  the  tube  into  the 
main  tube.  By  means  of  the  rod  the  stopper  was  readily  pushed  into  place. 
This  botthng  apparatus  in  its  improved  form  was  first  used  by  Richards  and 
Parker  in  their  work  upon  magnesium  chloride.^  The  bottle  was  transferred  to 
a  desiccator  and,  after  standing  near  the  balance  case  for  some  time,  it  was 
weighed  by  substitution  for  a  counterpoise  similar  in  weight  and  volume  as 
well  as  shape. 

Sublimation  of  the  cadmium  chloride  always  took  place  to  some  extent  during 
the  fusion,  and  the  sublimed  salt  occasioned  some  difficulty  since  it  flowed  down 
the  inside  of  the  glass  tube  and,  upon  soHdification,  firmly  cemented  the  boat 
to  the  tube.  Furthermore,  the  salt  which  adhered  to  the  outside  of  the  boat  had 
thus  been  fused  in  contact  with  glass,  and  hence  may  have  been  impure.  Both 
these  difficulties  were  avoided  by  supporting  the  boat  upon  a  carriage  of  heavy 
platimmi  wire.  While  the  salt  was  still  warm  and  the  current  of  hydrochloric- 
acid  gas  was  still  passing,  the  boat  was  pushed  out  of  the  carriage  by  means  of 
a  long  glass  rod.  Neglect  to  observe  the  latter  precaution  usually  resulted  in 
the  cementing  of  the  boat  to  the  carriage  by  the  salt  which  had  condensed  upon 
the  outside  of  the  boat. 

It  has  already  been  shown  that  barium  and  calcium  chlorides  when  they  have 
been  fused  and  allowed  to  solidify  in  an  atmosphere  of  hydrochloric-acid  gas, 
occlude  none  of  the  gas,^  for  they  give  neutral  solutions;  hence  it  is  reasonable 
to  conclude  that  this  is  the  case  with  cadmium  chloride  also.  However,  in  order 
to  test  this  point,  in  analysis  9  the  hydrochloric  acid  was  displaced  by  nitrogen 
while  the  salt  was  still  warm,  and  in  analysis  8  the  salt  was  allowed  to  solidify 
only  when  the  hydrochloric  acid  had  been  almost  completely  displaced  by 
nitrogen.  In  one  experiment  where  the  hydrochloric  acid  had  been  completely 
displaced  by  nitrogen,  the  boat  became  covered  with  a  gray  coating  which 
turned  brown,  and  finally  volatilized  when  the  boat  was  ignited.  This  coating 
undoubtedly  consisted  of  metallic  cadmium,  formed  either  by  the  dissociation 
of  cadmium  chloride  vapor  or  by  the  action  of  the  small  amount  of  hydrogen  con- 
tained in  nitrogen  produced  by  Wanklyn's  method,  owing  to  catalytic  decom- 
position of  the  excess  of  ammonia  by  the  hot  copper.  The  close  agreement  of  the 
results  of  analyses  8  and  9  with  those  obtained  in  the  other  analyses  where  the 
salt  solidified  and  cooled  in  hydrochloric  acid,  shows  conclusively  that  no  ap- 
preciable amount  of  hydrochloric  acid  was  occluded  by  the  salt. 

*  Loc.  cit. 

^  Richards:  Proc.  Amer.  Acad.,2g,  59  (1893);  Ze«^  anorg.  Chem.,  6, 93  (1894);  Jour.  Amer. 
Chem.  Soc,  27,  376  (1902);  Zeit.  anorg.  Chem.,  31,  273. 


lO  RESEARCHES   UPON   ATOMIC  WEIGHTS. 

It  was  shown  in  our  determination  of  the  specific  gravity  of  cadmium  chloride, 
that  the  salt  when  prepared  in  this  way  contains  no  ammonium  chloride.  It 
is  probable  that  the  cadmium  chloride  contained  no  basic  compound,  since  no 
insoluble  salt  was  produced  when  the  chloride  was  dissolved  in  water.  The 
aqueous  solution  of  the  salt  invariably  contained  a  few  tenths  of  a  milligram  of 
black  insoluble  matter  which  consisted  chiefly  of  platinum.  The  presence  of 
this  platinum  was  undoubtedly  due  partially  to  slight  attacking  of  the  boat, 
owing  perhaps  to  contamination  of  the  hydrochloric  acid  with  traces  of  air. 
The  slight  loss  in  weight  of  the  boat  which  resulted  in  most  of  the  analyses  was 
not  sufficient  to  accoimt  for  all  the  insoluble  residue,  which,  therefore,  must 
have  had  its  source,  in  part,  in  the  original  material.  Whether  the  platinum 
was  dissolved  from  the  platinum  condenser  during  the  distillation  of  the  hydro- 
chloric acid,  or  from  the  platinum  dish  during  the  solution  of  the  cadmiiun  is 
uncertain.  At  all  events,  the  temperature  to  which  the  salt  was  heated  must 
have  been  sufficient  to  decompose  all  the  platinic  or  platinous  chlorides  present, 
and  since  the  insoluble  residue  was  filtered  out  and  weighed,  and  corrections 
applied  to  the  weight  of  the  salt  both  for  the  loss  in  weight  of  the  boat  and  for 
the  insoluble  matter,  no  appreciable  error  could  have  been  introduced  by 
the  platinum. 

THE   METHOD   OF   ANALYSIS. 

After  the  salt  had  been  weighed,  the  boat  was  transferred  to  a  flask  and  the 
salt  was  dissolved  in  about  200  c.c.  of  the  purest  water.  The  weighing-bottle 
was  rinsed  and  the  rinsings  were  added  to  the  solution.  Next  the  solution  was 
filtered  into  the  precipitating  flask  through  a  tiny  filter  to  collect  the  insoluble 
matter.  Filter-paper  and  residue  were  then  ignited  in  a  weighed  porcelain 
crucible. 

In  the  preliminary  analyses  the  ratio  of  cadmium  chloride  to  silver  chloride 
was  determined  by  adding  to  the  solution  of  cadmium  chloride,  which  had  been 
diluted  in  the  (Erlenmeyer)  precipitating  flask  until  not  stronger  than  i  per 
cent,  a  solution  of  a  slight  excess  of  silver  nitrate  of  very  nearly  the  same  con- 
centration. The  flask,  which  was  provided  with  a  ground-glass  stopper,  was 
shaken  for  some  time,  and  was  allowed  to  stand  until  the  solution  was  clear. 
Then  the  precipitate  of  silver  chloride  was  transferred  to  a  Gooch  crucible,  after 
it  had  been  washed  by  decantation  six  or  eight  times  with  about  150  c.c.  of  a 
0.00 1  normal  silver  nitrate  solution,  and  finally  several  times  with  pure  water. 
Needless  to  say,  the  operations  of  precipitation  and  filtration  were  performed 
in  a  room  hghted  only  with  ruby  light.  The  crucible  with  the  precipitate  was 
placed  in  an  air-bath  and  heated  for  several  hours  at  130°  to  140°  C,  and  after 
it  had  cooled  in  a  desiccator  it  was  weighed.  In  order  to  determine  how  much 
moisture  was  retained  by  the  precipitate  in  each  case,  it  was  transferred  to  a 
clean  porcelain  crucible  and  weighed,  then  the  salt  was  fused  by  heating  the 
small  crucible,  contained  in  a  larger  covered  crucible,  and  again  weighed.    Two 


THE   ANALYSIS   OF   CADMIUM   CHLORIDE.  II 

different  specimens  of  silver  chloride  from  analyses  were  separately  dissolved  in 
ammonia  and  reprecipitated  with  hydrochloric  acid,  and  the  filtrates,  after  evap- 
oration, were  tested  for  cadmium.  Negative  results  were  obtained  in  both 
cases,  showing  both  that  silver  chloride  does  not  occlude  cadmium  salts  to 
an  appreciable  extent  and  that  the  washing  of  the  silver  chloride  had  been 
thorough. 

The  determination  of  the  silver  chloride  dissolved  in  the  wash-waters  was 
the  most  difficult  step  in  the  analysis.  At  first  the  last  few  washings,  those  which 
had  been  carried  out  with  pure  water  and  which  were  the  only  ones  which  could 
have  contained  dissolved  silver  chloride,  were  evaporated  to  small  bulk  and  an 
excess  of  silver  nitrate  was  added.  The  precipitate  of  silver  chloride,  together 
with  any  asbestos  which  had  been  displaced  from  the  Gooch  crucible,  was 
collected  upon  a  small  filter  which  was  ignited  and  weighed.  Owing  to  the  com- 
bined effect  of  organic  matter  and  light  upon  these  solutions  the  precipitate  was 
always  too  heavy.  Hence  this  method  was  finally  discarded.  Four  preliminary 
results  obtained  in  this  way  varied  between  112.34  and  11 2.40  for  the  atomic 
weight  of  cadmium. 

In  order  to  avoid  this  error,  in  the  next  series  the  silver  chloride  dissolved  in 
the  wash- waters  was  determined  by  precipitating  the  chloride  in  25  c.c. 
portions  of  the  solution  with  an  excess  of  silver  nitrate,  and  comparing  in  a 
nephelometer  the  precipitate  produced  with  that  from  solutions  prepared 
from  standard  hydrochloric-acid  solutions.  At  least  two  comparisons  were 
made  in  each  analysis. 

The  nephelometer  employed  for  the  estimation  of  slight  opalescences  has  al- 
ready been  described  in  detail  by  Richards  and  Wells.^  All  the  precautions  ne- 
cessary for  the  accurate  use  of  this  instrument  were  carefully  observed.  The 
two  tubes  to  be  compared  were  always  of  the  same  size.  The  source  of  light  in 
the  nephelometer  was  so  adjusted  that  tubes  containing  exactly  equal  amounts 
of  precipitate  gave  identical  readings.  It  was  found  advantageous  to  insert  a 
plate  of  ground  glass  between  the  source  of  light  and  the  test-tubes.  In  making 
up  the  test  solutions,  great  pains  were  taken  that  the  concentration  of  electro- 
lytes in  the  two  solutions  and  the  conditions  of  precipitation  should  be  as 
nearly  as  possible  the  same.  Final  readings  were  taken  only  after  the  ratio 
between  the  two  tubes  had  become  constant. 

The  weight  of  pure  silver  required  exactly  to  combine  with  the  chlorine  in 
cadmium  chloride  also  was  determined.  From  the  weight  of  cadmium  chloride 
very  nearly  the  necessary  quantity  of  pure  silver  was  calculated.  This  silver 
was  weighed  out  and  dissolved,  in  a  flask  provided  with  a  column  of  bulbs  to 
prevent  loss  of  silver  by  spattering  (see  fig.  2)  in  distilled  nitric  acid  diluted  with 
an  equal  volume  of  water.  Ordinarily  the  silver  was  caused  to  dissolve  so  slowly 
that  practically  no  gas  was  evolved.    Careful  experiments  have  shown,  how- 

^  Amer.  Chem.  Jour.,  31,  235  (1904);  35,  510  (1906). 


12 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


ever,  that  the  column  of  bulbs  is  ample  protection  against  loss  by  spattering, 
even  when  considerable  effervescence  takes  place  during  the  solution. 

After  the  silver  was  dissolved,  the  solution  was  diluted  somewhat  and  heated 
until  free  from  nitrous  fumes.  Then  it  was  further  diluted  until  not  more  con- 
centrated than  I  per  cent,  and  was  slowly  added  to  the  i  per  cent  solution  of 
cadmium  chloride  in  the  precipitating  flask.  After  sev- 
eral minutes'  shaking  it  was  allowed  to  stand  several  days, 
with  occasional  shaking,  until  the  solution  was  perfectly 
clear.  Two  30  c.c.  portions  of  the  clear  liquid  were  then 
pipetted  into  test  tubes  of  similar  size.  To  one  portion 
was  added  i  mg.  of  silver  nitrate  in  the  form  of  hundredth 
normal  solution,  to  the  other  an  equivalent  amount  of 
hydrochloric-acid  solution,  and  the  tubes  were  examined 
at  frequent  intervals  in  the  nephelometer  until  the  ratio 
of  the  opalescence  shown  by  the  two  tubes  became 
constant.  Richards  and  Wells  have  shown  that  when 
equivalent  amounts  of  silver  and  chloride  have  been  used 
in  the  original  precipitation,  the  nephelometer  tubes  show 
equal  opalescence.  If  this  was  not  the  case  in  the  first 
examination,  the  contents  of  the  tubes  were  returned  to 
the  precipitating  flask  and  either  standard  silver  nitrate 
solution  or  standard  hydrochloric-acid  solution  was  added 
and  the  shaking  and  testing  repeated  until  the  amounts  of 
chloride  and  silver  in  the  solution  were  equivalent. 

FinaUy  a  considerable  excess  of  silver  nitrate  was 
added  to  the  analysis  to  precipitate  dissolved  silver 
chloride,  and  the  silver  chloride  was  determined  gravimetrically  as  previously 
described.  Correction  was  of  course  made  for  chloride  introduced  in  the  course 
of  the  nephelometric  tests. 

A  vacuum  correction  of  -l- 0.000152  gm.  was  applied  for  every  apparent  gram 
of  cadmium  chloride,  of  -H  0.000071  gm.  for  every  apparent  gram  of  silver  chlo- 
ride, and  of  —0.000031  gm.  for  every  apparent  gram  of  silver.^ 

All  weighings  were  made  by  substitution,  with  tare  vessels  as  nearly  as  pos- 
sible Hke  those  weighed.  The  gold-plated  brass  weights  were  twice  carefully 
standardized  to  hundredths  of  a  milligram. 


Fig.  2.  —  Flask  for 
dissolving  silver. 


^  The  specific  gravity  of  cadmium  chloride  has  been  found  to  be  4.047.  Baxter  and  Hines: 
Amer.  Chem.  Jour.,  31,  220  (1904).  Richards  and  StuU  have  determined  the  specific  grav- 
ity of  silver  chloride  to  be  5.56,  and  Richards  and  Wells  that  of  silver  to  be  10.49.  Pub.  Car, 
Inst.,  No.  28,  II  (1905);  Jour.  Amer.  Chem.  Sac,  27, 466;   Zeit.  anorg.  Chem.,  47, 64. 

The  specific  gravity  of  the  weights  is  assumed  to  be  8.3.  (See  page  40.)  The  use  of  this 
low  value  for  the  specific  gravity  of  the  weights  has  led  to  slight  changes  in  the  vacuum  cor- 
rections of  cadmium  and  silver  chlorides  from  the  values  used  in  the  original  publication  of 
this  paper. 


THE   ANALYSIS   OF   CADMIUM   CHLORIDE. 


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14  RESEARCHES    UPON    ATOMIC   WEIGHTS. 

The  close  agreement  of  the  results  in  each  series  leaves  little  doubt  of  the 
identity  of  the  different  samples,  although  they  represent  material  from  different 
sources  as  well  as  different  fractions  of  the  same  material.  The  slight  discrep- 
ancy between  the  results  by  the  two  methods  is  undoubtedly  due,  in  part,  to 
the  difl&culty  in  determining  silver  chloride  with  accuracy,  owing,  in  the  first 
place,  to  loss  of  chlorine  by  the  silver  chloride  in  the  processes  of  manipulation 
and  drying,  and  in  the  second  place  to  the  slight  solubility  of  silver  chloride 
even  in  dilute  silver  nitrate  solutions.  In  the  light  of  these  possibilities,  it  is 
probable  that  the  results  of  Series  I  are  slightly  too  high.  On  the  other  hand,  it 
is  probable  that  the  average  of  Series  II  is  slightly  too  low,  for  the  average  of 
experiments  7,  8  and  9,  in  which  the  experience  gained  in  the  previous  analyses 
was  a  very  considerable  aid,  is  112.415,  0.006  of  a  unit  higher  than  the  average 
of  the  whole  series. 

ACTION  OF  HYDROCHLORIC-ACID  GAS  UPON  PHOSPHORUS  PENTOXIDE. 

Some  time  after  the  completion  of  the  foregoing  series  of  experiments  it  was 
found  that  in  fusing  manganous  chloride,  in  a  current  of  hydrochloric-acid  gas 
which  had  been  dried  by  concentrated  sulphuric  acid  and  finally  by  means  of 
phosphorus  pentoxide,  an  insoluble  residue  of  manganous  phosphate  was  in- 
variably obtained  when  the  salt  was  dissolved  in  water.    The  quantity  of  this 
residue  varied  with  the  amount  of  moisture  contained  by  the  salt  when  brought 
in  contact  with  the  hydrochloric-acid  gas,  being  extremely  slight  if  the  salt  was 
very  nearly  dry,  but  amounting  to  several  milligrams  if  the  salt  still  contained 
much  of  its  crystal  water.    Although  it  seemed  certain  that  the  phosphorus  had 
its  origin  in  the  phosphorus  pentoxide,  and  was  volatilized  in  the  form  of  either 
phosphorus  pentachloride  or  oxychloride  through  the  action  of  the  hydro- 
chloric acid  upon  the  pentoxide,  in  order  to  obtain  still  more  positive  evidence 
that  this  was  really  the  case,  the  experiment  was  tried  of  passing  hydrochloric- 
acid  gas  which  had  been  dried  thoroughly  by  means  of  sulphuric  acid,  first 
over  phosphorus  pentoxide  which  had  been  freshly  sublimed  in  a  current  of 
dry  air,  and  then  into  water.    The  aqueous  solution,  upon  evaporation  and 
testing  with  ammoniimi  molybdate,  gave  a  considerable  amount  of  the  charac- 
teristic ammonium  phosphomolybdate.    This  result  confirms  that  of  Bailey 
and  Fowler,^  who  have  fovmd  that  both  hydrochloric  and  hydrobromic  acids 
react  with  phosphorus  pentoxide  at  ordinary  temperatures  to  form  the  oxy- 
chloride and  bromide  of  phosphorus  respectively.    The  manganous  phosphate, 
then,  must  have  been  produced  by  the  action  of  the  volatilized  chloride  of 
phosphorus  upon  the  moisture  contained  by  the  manganous  chloride  to  form 
phosphoric  acid,  with  subsequent  displacement  of  hydrochloric  acid  from  the 
salt  by  the  phosphoric  acid. 

*  Chem.  News,  58,  22  (i 


THE   ANALYSIS   OF   CADMIUM  CHLORIDE.  1 5 

Although  in  our  work  with  cadmium  chloride,  the  double  cadmium  ammon- 
ium chloride  was  fused  in  a  current  of  hydrochloric-acid  gas  which  had  been 
finally  dried  with  phosphorus  pentoxide,  the  salt,  which  contains  no  crystal 
water,  was  essentially  free  from  moisture  before  coming  in  contact  with  the 
hydrochloric  acid.  Nevertheless  it  seemed  desirable  to  repeat  the  experiments 
with  cadmivun  chloride  in  such  a  way  that  the  danger  mentioned  above, could 
be  completely  avoided.  This  result  was  easily  attained  by  drying  the  hydro- 
chloric-acid gas  with  concentrated  sulphuric  acid  only. 

In  order  to  ascertain  whether  concentrated  sulphuric  acid  is  appreciably  at- 
tacked by  hydrochloric-acid  gas,  a  large  quantity  of  this  gas  was  conducted 
through  the  columns  and  then  into  water.  The  aqueous  solution  was  then 
evaporated  and  tested  for  sulphate  with  barium  chloride.  Although  a  slight 
precipitate  of  baric  sulphate  was  produced,  the  quantity  was  estimated,  by 
comparison  in  a  nephelometer  with  a  standard  solution  of  a  sulphate,  to  be  less 
than  0.05  mg.    Evidently  nothing  is  to  be  feared  from  this  source. 

PREPARATION   AND   DRYING   OF   CADMIUM   CHLORIDE 
FOR   THE   FINAL  ANALYSES. 

The  material  for  these  experiments  was  prepared  from  a  portion  of  fraction 
II  of  cadmimn  sulphide,  by  first  depositing  the  metal  electrolytically  from  the 
sulphate.  At  first  electrolysis  was  carried  on  in  a  solution  of  the  sulphate  in 
pure  water,  between  two  electrodes  of  platinum  foil.  A  sponge  of  extremely 
small  crystals  was  thus  produced.  These  crystals  contained  occluded  sulphate 
in  considerable  quantities,  and  no  amount  of  washing  with  water  was  sufficient 
completely  to  leach  out  this  occluded  material.  More  satisfactory  results  were 
obtained  by  depositing  the  metal  upon  a  platinum  dish  which  had  been  covered 
with  a  very  thin  film  of  soft  paraffine,  so  that  the  deposit  could  be  readily  sep- 
arated from  the  dish.^  The  cadmium  was  first  washed  with  water,  then  with 
ether,  next  with  alcohol,  and  finally  with  water  again.  This  treatment  effect- 
ually cleansed  the  metal  from  paraffine. 

The  metal  was  next  dissolved  in  pure  hydrochloric  acid  in  a  platinum  dish. 
The  chloride  does  not  lend  itself  readily  to  crystallization  from  aqueous  solu- 
tion on  account  of  its  great  solubihty  even  at  low  temperatures,  but  by  con- 
ducting hydrochloric-acid  gas  into  the  solution  the  much  less  soluble  double 
salt  with  hydrochloric  acid,  CdCl22HCl7H20,  was  formed.  The  salt  was  thus 
crystallized  3  times  with  centrifugal  drainage,  to  free  it  from  the  trace  of  sul- 
phates occluded  by  the  metal  during  electrolysis.  Finally,  it  was  dried  and 
freed  from  hydrochloric  acid  as  far  as  possible  in  a  vacuum  desiccator  contain- 
ing solid  potassium  hydroxide. 

Before  fusion  the  salt  was  largely  freed  from  water  and  hydrochloric  acid  by 
gentle  heating. 

^  Richards:  Proc.  Amer.  Acad. ,2$,  200  (iSgo). 


i6 


RESEARCHES    UPON    ATOMIC    WEIGHTS. 


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THE   ANALYSIS   OF   CADMIUM   CHLORIDE.  1 7 

The  average  of  these  results  is  almost  identical  with  that  obtained  in  the 
first  two  series  of  analyses,  11 2.4 17,  hence  it  is  evident  that  no  serious  error 
was  introduced  in  our  earher  work  by  the  use  of  phosphorus  pentoxide 
for  drying  hydrochloric-acid  gas. 

The  results  of  this  investigation  may  be  summarized  as  follows: 

1.  In  the  analysis  of  cadmium  chloride,  both  gravimetrically  by  determina- 
tion of  the  chlorine  as  silver  chloride  and  volumetrically  by  comparison  with 
silver,  the  atomic  weight  of  cadmium  is  foimd  to  be  11 2.418  referred  to  silver 
107.880,  or  112.408  if  silver  has  the  atomic  weight  107.870. 

2.  Phosphorus  pentoxide  is  found  to  be  attacked  by  pure  hydrochloric-acid 
gas,  and  hence  is  unsuited  for  drying  this  gas,  thus  confirming  the  results  of 
Bailey  and  Fowler, 

3 .  It  is  shown  that  no  appreciable  error  is  introduced  from  this  source,  if  a  dry 
salt  is  fused  in  a  current  of  hydrochloric  acid  which  has  been  dried  by  phospho- 
rus pentoxide. 


II. 

A     REVISION     OF     THE     ATOMIC     WEIGHT 
OF   CADMIUM. 

THE  ANALYSIS   OF  CADMIUM  BROMIDE. 


By  Gregory  Paul  Baxter,  Murray  Arnold  Hines,  and  Harry  Louis  Frevert. 


Journal  of  the  American  Chemical  Society,  28,  770  (1906). 

Zeitschrift  fiir  anorganische  Chemie,  49,  415  (1906). 

Chemical  News,  94,  224,  236,  248  (1906). 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF  CADMIUM. 
THE   ANALYSIS   OF   CADMIUM  BROMIDE. 


Since  the  research  described  in  the  preceding  paper  indicates  that  the  atomic 
weight  of  cadmium  is  nearly  one  tenth  of  a  unit  higher  than  the  results  of 
recent  prior  determinations  by  other  investigators,  in  order  to  confirm  or  dis- 
prove the  higher  value  the  analysis  of  cadmium  bromide  was  undertaken. 


PURIFICATION   OF   MATERIALS. 


CADMIUM    BROMIDE. 


The  cadmium  material  employed  for  the  work  consisted  of  fractions  II  and 
III  of  sulphide,  which  were  obtained  in  the  earlier  investigation  (page  4). 
Here  also  the  sulphide  was  first  converted  into  sulphate,  and  after  crystalli- 
zation of  the  sulphate  the  cadmium  was  deposited  electrolytically  upon  a 
platinum  dish  which  had  been  coated  with  a  thin  film  of  paraflfine.  The 
deposit  was  separated  from  the  dish  mechanically  and  after  washing  with  water 
was  freed  from  paraffine  with  redistilled  ether  and  alcohol.  By  the  method 
finally  adopted  for  converting  the  cadmium  into  bromide,  it  was  covered  in  a 
quartz  dish  with  water  slightly  acidified  with  hydrobromic  acid  to  prevent  the 
formation  of  basic  cadmium  salts,  and  the  purest  bromine  was  added  in  small 
quantities  until  the  metal  was  almost  wholly  dissolved.  The  solution  was 
heated  with  the  residual  metallic  cadmium  upon  a  steam-bath  until  every 
trace  of  bromine  had  disappeared.  Then  it  was  filtered  with  a  platinmn 
funnel  into  a  platinimi  dish,  and  was  recrystalHzed  three  times,  with  centrif- 
ugal drainage  in  the  platinum  funnel  after  each  crystalUzation.^  The  original 
solution  contained  only  traces  of  sulphate,  and,  when  tested  with  barium  hy- 
droxide, the  mother-Uquors  of  the  third  crystallization  gave  absolutely  no  test 
for  sulphate,  hence  the  crystals  themselves  must  have  been  pure  (Samples  II 
and  III).  The  crystals  were  dried  over  potassium  hydroxide  in  a  vacuum 
desiccator. 

BROMINE. 

Commercial  bromine  was  freed  from  chlorine  by  two  distillations  from  a  con- 
centrated solution  of  a  bromide,  the  bromide  in  the  second  distillation  being 
almost  free  from  chloride.  The  bromine  was  covered  with  water,  and  hydrogen 
sulphide,  which  had  been  thoroughly  washed  with  water,  was  passed  into  the 

^  Richards:  Jour.  Amer.  Chem.  Soc,  27,  no  (1905). 

21 


22  RESEARCHES    UPON    ATOMIC   WEIGHTS. 

solution  until  reduction  of  the  bromine  was  complete.  The  solution  was  boiled, 
after  mechanical  separation  of  the  greater  part  of  the  free  sulphur  and  bromide 
of  sulphur,  and  was  filtered.  Iodine  was  eliminated  by  boiling  the  hydrobromic 
acid  with  several  small  portions  of  potassium  permanganate  and  rejecting  the 
bromine  set  free.  By  heating  the  remainder  of  the  hydrobromic  acid  with  an 
excess  of  permanganate,  over  half  of  the  bromine  was  obtained  in  the  free  state. 
The  process  of  reduction  with  hydrogen  sulphide  and  oxidation  with  perman- 
ganate was  then  repeated  with  the  resulting  bromine,  and  the  final  product  was 
redistilled  shortly  before  use. 

SILVER. 

One  sample  of  silver  was  purified  especially  for  this  research.  The  processes 
to  which  it  was  subjected  consisted  first  of  precipitation  as  chloride  from  nitric- 
acid  solution  and  reduction  with  invert  sugar  and  sodiimi  hydroxide.  After 
fusion  with  a  blowpipe  on  a  crucible  of  the  purest  lime  the  metallic  buttons 
were  freed  from  surface  impurities  by  scrubbing  with  moist  sand  and  etching 
with  nitric  acid.  Next  the  buttons  were  dissolved  in  nitric  acid  and  the  solu- 
tion was  reduced  with  ammonium  formate.^  The  precipitated  silver  was 
thoroughly  washed  and  again  fused  on  a  Ume  crucible.  The  final  process  of 
purification  consisted  in  electrolyzing  the  silver  as  described  on  page  6.  The 
electrolytic  crystals  were  fused  in  a  current  of  hydrogen  on  a  lime  boat,  and 
the  buttons,  after  cleansing  with  nitric  acid  and  dr5dng  at  200°,  were  cut  into 
fragments  of  convenient  size  with  a  clean  chisel  and  anvil.  Then  they  were  again 
treated  with  fresh  portions  of  dilute  nitric  acid  until  free  from  iron,  washed, 
dried,  and  finally  heated  to  about  400°  in  a  vacuum.  This  silver  was  employed 
in  analyses  4  to  8. 

In  the  first  three  analyses  a  mixture  of  two  specimens  of  silver  was  em- 
ployed, both  of  which  had  already  been  used  in  an  investigation  upon  the 
atomic  weight  of  iodine  by  one  of  us.^  One  was  prepared  from  silver  nitrate 
which  had  been  seven  times  recrystalhzed  from  nitric  acid,  five  times  recrys- 
talHzed  from  water,  and  finally  precipitated  with  ammonium  formate.  The 
other  was  precipitated  once  as  silver  chloride,  electrolyzed  once,  and  finally 
reduced  with  ammonium  formate. 

DRYING   OF   CADMIUM    BROMIDE   FOR   ANALYSIS. 

The  method  of  analysis  was  essentially  that  usually  employed  in  this 
laboratory  for  the  analysis  of  metalHc  haUdes.  Weighed  portions  of  the 
bromide,  after  fusion  in  nitrogen  and  hydrobromic-acid  gases,  were  first  titrated 
against  weighed  portions  of  silver.  Then  the  precipitated  silver  bromide  was 
collected  and  weighed. 


^  Richards:  Pub.  Car.  Inst.  No.  28,  p.  19  (1905);  Jour.  Amer.  Chem.  Soc,  27,  475;  Zeit- 
anorg.  Chem.,  47,  72, 

2  Baxter:  Proc.  Amer.  Acad..  41,  79  (1905);  Jour.  Amer.  Chem.  Soc. ,27,  881;  Zeit.  anorg. 
Chem.,  46,  42.    (See  page  108.) 


THE   ANALYSIS   OF    CADMIUM   BROMIDE. 


23 


The  apparatus  used  for  the  fusion  of  the  salt  in  nitrogen  and  hydrobromic- 
acid  gases  was  employed  in  the  preparation  of  ferrous  bromide  by  one  of  us/ 
and  is  a  modification  of  apparatus  used  for  a  similar  purpose  in  the  determina- 
tion of  the  atomic  weight  of  cobalt,^  in  this  laboratory.  Nitrogen  was  prepared 
by  passing  air  through  concentrated  ammonia  solution  in  the  bottle  F  (fig.  3) 
and  then  over  hot  copper  gauze  in  the  hard-glass  tube  G.    The  excess  of  am- 


FiG.  3.  —  Apparatus  for  the  fusion  of  bromides  in  a  current  of  dry  nitrogen 
and  hydrobromic-acid  gases. 

monia  was  removed  by  dilute  sulphuric  acid  in  the  bottles  H  and  I.  The  gas 
was  then  conducted  into  an  apparatus  constructed  wholly  of  glass,  with  ground 
joints,  which  consisted  of  a  tower,  J,  filled  with  beads  moistened  with  silver  ni- 
trate solution  to  remove  sulphur  compounds,  two  similar  towers,  K  and  L,  con- 
taining dilute  sulphuric  acid  to  ehminate  last  traces  of  ammonia,  and  two 
towers,  M  and  N,  filled  with  sticks  of  fused  potassiimi  hydroxide  to  absorb 
moisture  and  carbon  dioxide.  The  partially  dried  gas,  after  bubbling  through 
bromine  in  a  small  flask,  P,  passed  into  a  second  flask,  Q,  containing  concentrated 
hydrobromic-acid  solution  in  which  washed  red  phosphorus  was  suspended,  to 
convert  the  bromine  into  hydrobromic  acid.  A  U-tube,  R,  also  containing  red 
phosphorus  and  hydrobromic  acid,  removed  traces  of  bromine  which  escaped 
reduction  in  the  flask.  Two  additional  U -tubes,  S  and  T,  containing  beads 
moistened  with  concentrated  hydrobromic  acid  only,  served  to  eliminate  phos- 
phorus compounds  which  were  found,  in  the  investigation  upon  ferrous  bro- 
mide,^ to  accompanji'  the  hydrobromic  acid  if  the  phosphorous  acid  in  the  reduc- 
tion flask  was  allowed  to  become  very  concentrated.    Finally,  the  mixture  of 


1  Baxter:  Proc.  Amer.  Acad.,  39,  246  (1903);  Zeit.  anorg.  Chem.,  38,  233  (1904). 

2  Richards  and  Baxter:  Proc. ^raer.^cac?., 33, 117  (1897);  Zeit. anorg.Chem., 16, 572  (i 
'  Loc.  cit. 


24  RESEARCHES   UPON  ATOMIC   WEIGHTS. 

nitrogen  and  hydrobromic-acid  gases  was  thoroughly  dried,  first  by  pure  fused 
calcium  bromide,  in  the  tube  U,  and  then  by  resublimed  phosphorus  pentoxide 
in  the  tube  W. 

If  desired  the  nitrogen  could  be  passed  directly  through  the  phosphorus  pent- 
oxide  tube  O  into  the  botthng  apparatus  XY,  in  which  the  fusion  took  place. 

Air  was  purified  and  dried  by  passing  over  fused  potassium  hydroxide  in 
the  tower  A,  concentrated  sulphuric  acid  in  the  towers  B  and  C,  and  phosphorus 
pentoxide  in  the  tubes  D  and  E. 

The  cadmium  bromide,  contained  in  a  weighed  platinum  boat,  was  heated 
gently  in  a  current  of  nitrogen  until  a  small  quantity  of  residual  crystal  water 
was  expelled,  then  strongly  in  a  current  of  nitrogen  and  hydrobromic  acid  imtil 
fused.  After  the  salt  had  cooled,  the  hydrobromic  acid  was  displaced  by  nitro- 
gen and  this  in  turn  by  dry  air.  During  the  displacement  of  the  hydrobromic 
acid  by  nitrogen  and  air  a  shght  backward  current  of  gas  was  maintained 
through  the  tube  W  and  the  trap  V.  The  boat  was  then  transferred  to  the 
weighing-bottle  in  which  it  was  originally  weighed,  and  the  stopper  was  inserted 
without  an  instant's  exposure  of  the  salt  to  moisture,  by  means  of  the  bottling 
apparatus  which  has  been  referred  to  on  page  9.  The  weighing-bottle  was 
then  allowed  to  stand  in  a  desiccator  near  the  balance  case  for  some  time  before 
it  was  weighed. 

METHOD   OF   ANALYSIS. 

Next  the  boat  was  transfered  to  a  flask  and  the  salt  was  dissolved  in  about 
300  c.c.  of  the  purest  water.  The  weighing-bottle  was  rinsed  and  the  rinsings 
were  added  to  the  solution.  Then  the  solution  was  filtered  into  the  glass-stop- 
pered precipitating  flask  through  a  tiny  filter  to  collect  a  trace  of  insoluble 
matter,  and  the  filter-paper  and  residue  were  ignited  at  a  low  temperature  in  a 
weighed  porcelain  crucible.  This  residue,  which  usually  amounted  to  less  than 
0.1  mg.  and  was  never  as  much  as  0.2  mg.,  did  not  contain  detectable  quantities 
of  cadmium,  and  probably  consisted  of  silica  and  a  trace  of  platinum  re- 
moved from  the  boat  during  the  fusion,  for  the  boat,  when  reweighed,  in  most 
cases  was  found  to  have  lost  a  few  hundredths  of  a  milligram.  No  change  in 
weight  could  be  found  when  the  boat  was  first  dried  and  weighed,  then  ignited 
and  reweighed.  The  difference  between  the  weight  of  the  residue  and  the  loss 
in  weight  of  the  boat  was  subtracted  from  the  weight  of  the  cadmium 
bromide. 

From  the  corrected  weight  of  the  cadmium  bromide  very  nearly  the  requisite 
quantity  of  pure  silver  was  calculated.  This  silver  was  weighed  out  and  dis- 
solved, in  nitric  acid  diluted  with  an  equal  volimie  of  water,  in  the  flask  de- 
scribed on  page  12.  After  the  silver  was  dissolved,  the  solution  was  diluted  to 
twice  its  volume  and  was  heated  until  free  from  nitrous  fumes.  Then  it  was  still 
further  diluted  until  not  stronger  than  i  per  cent,  and  was  slowly  added,  with 
constant  stirring,  to  the  i  per  cent  solution  of  cadmium  bromide  in  the  precipi- 


THE    ANALYSIS    OF    CADMIUM    BROMIDE.  25 

tating  flask.  In  three  analyses  (4,  5  and  8),  this  procedure  was  varied  by 
adding  the  bromide  to  the  silver  nitrate.  After  being  shaken  for  some  time,  the 
solution  was  allowed  to  stand  several  days,  with  occasional  shaking,  until  the 
supernatant  liquid  was  clear.  30  c.c.  portions  of  the  solution  were  then  tested 
with  hundredth  normal  solutions  of  silver  nitrate  and  sodium  bromide  in  the 
nephelometer  ^  for  excess  of  bromide  or  silver,  and,  if  necessary,  either  standard 
silver  nitrate  or  sodium  bromide  solution  was  added,  and  the  process  of  shaking 
and  testing  repeated,  until  the  amounts  of  bromide  and  silver  in  the  solution 
were  equivalent.  If  the  solution  was  perfectly  clear  when  tested,  and  contained 
no  considerable  excess  of  bromide  or  silver,  the  test  solutions  were  discarded, 
since  they  contained  only  negligible  amounts  of  dissolved  silver  bromide;  other- 
wise they  were  returned  to  the  flask  and  a  correction  was  applied  for  the  silver 
bromide  thus  introduced. 

As  soon  as  the  exact  end-point  of  the  titration  had  been  found,  about  4  eg. 
of  silver  nitrate  in  excess  were  added,  to  precipitate  dissolved  silver  bromide, 
and  the  flask  was  again  shaken  and  allowed  to  stand  until  clear.  The  precipi- 
tate of  silver  bromide  was  collected  upon  a  weighed  Gooch  crucible,  after  it  had 
been  v/ashed  by  decantation  about  eight  times  with  pure  water.  Then  it  was 
heated  in  an  electric  air-bath,  first  for  several  hours  at  140°,  finally  for  an  hour 
at  200°,  and,  after  it  had  cooled  in  a  desiccator,  it  was  weighed.  In  order  to 
determine  how  much  moisture  was  retained  by  the  precipitate,  in  each  case  it 
was  transferred  as  completely  as  possible  to  a  clean  porcelain  crucible  and 
weighed;  then  the  salt  was  fused  by  heating  the  small  covered  crucible,  con- 
tained in  a  large  crucible,  and  again  weighed.  After  fusion  the  silver  bromide 
was  light  yellow,  with  only  a  trace  of  darkening,  showing  that  no  appreciable 
reduction  had  taken  place.  The  asbestos  mechanically  detached  from  the 
Gooch  crucible,  together  with  a  minute  quantity  of  silver  bromide  which  oc- 
casionally escaped  the  crucible,  was  collected  from  the  filtrate  and  wash-waters 
upon  a  small  filter,  the  ash  of  which  was  treated  with  nitric  and  hydrobromic 
acids  before  weighing.  Although  the  filtrates  and  first  wash-waters  were  essen- 
tially free  from  dissolved  silver  bromide,  the  subsequent  wash-waters  usually 
contained  a  trace  of  this  substance.  The  amount  of  dissolved  salt  was  deter- 
mined with  the  nephelometer  by  comparison  vnth.  standard  bromide  solutions. 
Finally,  the  weight  of  silver  bromide  was  corrected  for  the  sodium  bromide 
introduced. 

Although  in  our  analyses  of  cadmium  chloride  no  evidence  could  be  obtained 
of  appreciable  occlusion  of  either  cadmium  or  silver  salts  by  silver  chloride, 
especial  precautions  were  taken  to  avoid  any  possibihty  of  such  a  difficulty  in 
this  research.  In  the  first  place  both  the  cadmium  bromide  and  the  silver 
nitrate  solutions  were  very  dilute  during  precipitation,  each  one  having  a  vol- 
ume of  about  I  liter.    In  the  second  place  the  method  of  precipitation  was 

1  See  page  11. 


26  RESEARCHES  UPON  ATOMIC   WEIGHTS. 

varied  by  sometimes  adding  the  silver  nitrate  to  the  cadmium  bromide  (analyses 
I,  2,  3,  6,  and  7),  and  sometimes  adding  the  bromide  to  the  silver  nitrate 
(analyses  4,  5,  and  8).  And  in  the  third  place  the  solutions  were  allowed  to  stand 
varying  periods  before  the  titration  was  completed,  so  that  occluded  substances 
might  have  opportunity  to  be  dissolved.  Analysis  i,  in  which  the  largest  quan- 
tity of  bromide  was  employed,  over  11  gm.,  which  is  to  be  expected  to  give  the 
most  marked  evidences  of  occlusion,  was  not  tested  for  5  days  after  precipita- 
tion, and  the  titration  was  completed  8  days  later.  In  the  other  analyses  the 
period  between  precipitation  and  the  completion  of  the  titration  varied  from  7 
days  in  analysis  4  to  3  days  in  analysis  8.  Furthermore,  in  some  cases,  after  the 
end-point  had  been  reached,  the  solutions  were  allowed  to  stand  some  days 
longer  with  occasional  testing.  No  change  in  end-point  with  standing  was  ob- 
served. In  spite  of  these  differences  in  the  method  of  procedure,  the  variations 
in  the  final  results  do  not  exceed  the  experimental  error  to  be  expected,  except 
in  the  case  of  analyses  4  and  12.  Evidently,  occlusion  of  any  sort  must  have 
been  very  slight  if  it  existed  at  all.  Analyses  4  and  12,  performed  with  the 
same  portion  of  bromide,  differ  so  markedly  from  the  others  that,  although  no 
reason  for  the  difference  is  known,  they  are  rejected  in  computing  the  final 
average. 

The  gold-plated  brass  weights  were  carefully  standardized  to  hundredths  of 
a  milligram.  Vacuum  corrections  of  -}-o.oooo86  for  cadmium  bromide,^  of 
+0.000041  for  silver  bromide  and  of  —0.000031  for  silver  were  appUed.  All 
weighings  were  made  by  substitution  with  counterpoises  as  nearly  Uke  the  ob- 
jects to  be  weighed  as  possible. 

The  analytical  work  was  performed  wholly  by  Dr.  Hines.  (See  table  on 
opposite  page.) 

RESULTS   AND   DISCUSSION. 

The  ratios  of  silver  used  to  silver  bromide  obtained  in  the  same  analysis  afford 
sufficient  proof  of  the  purity  of  the  bromine  and  silver,  as  well  as  confirmatory 
evidence  of  the  absence  of  appreciable  occlusion  by  the  silver  bromide. 

Ag  :  AgBr. 

Analyses  i  and    9 57-4446 

2  "    10 57-4466 

3  "     II 57-4438 

4  "    12 57-4438 

s   "  13 57-4423 

6  "    14 57-4440 

7  "    15 57-4430 

8  "     16 57-4431 

Average,     57-4439 

^  The  specific  gravities  of  cadmium  and  silver  bromides  have  recently  been  found  to  be 
5.192  and  6.473  respectively.    Baxter  and  Hines:  Amer.  Chem.  Jour.,  3i»  220  (1904). 

As  in  the  case  of  cadmium  chloride,  slight  changes  in  the  vacuum  corrections  of  cadmium 
and  silver  bromides  have  been  made  from  the  values  employed  in  the  original  publication 
of  this  paper,  owing  to  more  exact  knowledge  of  the  density  of  the  weights. 


THE  ANALYSIS   OF   CADMIUM   BROMIDE. 


27 


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28  RESEARCHES    UPON    ATOMIC    WEIGHTS. 

The  most  probable  value  for  this  ratio  has  been  shown  by  Baxter  to  be 

574453-' 

In  the  preceding  paper  Bailey  and  Fowler's  ^  statement  that  hydrochloric- 
acid  gas  is  contaminated  with  volatile  compounds  of  phosphorus,  if  it  is  dried 
with  phosphorus  pentoxide,  is  confirmed.  Although  Bailey  and  Fowler  attrib- 
ute to  hydrobromic  acid  an  effect  similar  to  that  of  hydrochloric  acid,  for 
several  reasons  it  is  certain  that  in  the  experiments  upon  cadmium  bromide  no 
appreciable  amount  of  phosphorus  was  introduced  into  the  salt  by  the  action 
of  the  hydrobromic  acid  upon  the  phosphorus  pentoxide.  In  the  first  place,  the 
experiment  of  passing  into  water  hydrobromJc-acid  gas,  formed  as  in  our  work 
by  passing  nitrogen  through  bromine  and  then  through  an  emulsion  of  red 
phosphorus  in  concentrated  hydrobromic-acid  solution,  and  dried  first  by 
fused  calcium  bromide  and  then  by  phosphorus  pentoxide,  was  performed  in 
this  laboratory  some  years  ago  in  connection  with  the  analysis  of  cobalt  and 
nickel  bromides.  In  this  experiment  no  phosphorus  could  be  discovered  in  the 
aqueous  solution.  In  the  second  place,  in  two  analyses  of  bromides  which  had 
been  heated  in  hydrobromic-acid  gas,  the  filtrates  from  the  silver-bromide  pre- 
cipitates were  evaporated  to  small  bulk  and  tested  for  phosphoric  acid,  with 
negative  results,  while  the  slight  residues  obtained  by  filtering  the  aqueous  solu- 
tions of  the  original  bromides  also  showed  in  one  case  the  complete  absence  of 
phosphorus,  and  in  the  other  the  presence  of  only  a  minute  trace  of  this  sub- 
stance, although  in  the  latter  case  the  salt  had  been  sublimed  in  a  current  of 
hydrobromic  acid  and  therefore  contained  maximum  amounts  of  phosphorus.^ 
This  result  was  to  be  expected  from  a  consideration  of  the  fact  that  the  hydro- 
bromic-acid gas  used  in  these  experiments  was  diluted  with  at  least  twice  its 
volume  of  nitrogen.  In  the  light  of  this  evidence  it  seems  safe  to  assume  that 
in  the  numerous  analyses  of  bromides  which  have  been  carried  out  in  this  labo- 
ratory in  recent  years,  no  error  was  introduced  by  the  use  of  phosphorus  pent- 
oxide as  drying  agent  for  the  hydrobromic-acid  gas.  Nevertheless,  with  more 
concentrated  hydrobromic  acid,  doubtless  it  would  be  unwise  to  use  this  drying 
agent. 

It  is  interesting  to  compare  the  results  of  the  analyses  of  the  different  frac- 
tions of  material  in  this  research  and  in  the  preceding  one.  (See  table  on  op- 
posite page.) 

The  close  agreement  of  the  results  from  fraction  II  by  different  methods  and 
of  the  results  from  all  three  fractions  leaves  no  doubt  of  the  identity  of  the  dif- 
ferent specimens  of  material. 

No  matter  how  the  results  are  averaged,  the  same  conclusion  is  reached  as  in 
the  previous  paper,  i.  e.,  that  the  atomic  weight  of  cadmivun  hes  very  near  the 

*  Proc.  Amer.  Acad.,  42,  210  (1906);  Jour.  Amer.  Chem.  Soc,  28,  1332;  Zeit.  anorg.  Chem., 
50,  398.     (Seepage  51.) 

2  Chem.  News,  58,  22  (1888). 

3  Baxter:  Proc.  Amer.  Acad.,  39,  248  (1903);  Zeit.  anorg.  Chem.,  38,  236. 


THE   ANALYSIS    OP    CADMIUM   BROMIDE. 


29 


value  112.417  (Ag  =  107.880).    If  the  atomic  weight  of  silver  is  107.870,  then 
the  atomic  weight  of  cadmium  is  112.407. 


Fraction. 


I       .    .    . 
I       .    .    . 

II,  Series  i 
II,  Series  i 
II,  Series  2 
II,  Series  2 
II  .  .  . 
II     .    .    . 

Ill     ... 

Ill      ... 


Ratio. 


CdClz 

CdCla 

CdCla 

CdCl2 

CdCl2  : 

CdCl2  : 

CdBr2 

CdBra  : 

CdBra 

CdBra 


2Ag 

2AgCl 

2Ag 

2AgCl 

2Ag 

2AgCl 

2Ag 

2AgBr 

2Ag 

2AgBr 


Average 


Atomic 
weight. 


II2.415) 
I12.419  ) 
112.4031 
112.429  ) 
I12.418  1 
II2.418  ' 
I12.418  I 
112.414  ' 
112.4231 
112.413/ 


I12.417 


Average. 


112.417 
112. 416 
112.418 
112.416 
112.418 


112.417 


Attention  should  be  called  to  the  agreement  with  ours  of  the  results  of  Morse 
and  Arbuckle's  synthesis  of  cadmium  oxide,  which  )delded  the  value  112.38,  and 
of  Bucher's  painstaking  work  upon  the  halogen  compounds  of  cadmium. 
Bucher's  values  from  cadmium  chloride  vary  between  112. 21  and  112.41,  with 
an  average  of  112.32,  but  if  the  first  7  of  his  21  experiments  are  rejected,  his 
average  becomes  112.35,  and  6  of  his  results  are  as  high  as  112.38.  His  analyses 
of  the  bromide  vary  between  112.25  and  112.41,  with  an  average  of  112.34. 

The  results  of  this  investigation  are  then  as  follows: 

(i)  The  value  for  the  atomic  weight  of  cadmium  previously  found  by  analy- 
sis of  cadmium  chloride,  112.42  (Ag  =  107.880),  is  supported  by  the  analysis  of 
cadmimn  bromide.    If  silver  is  107.870,  cadmium  becomes  1 12.41. 

(2)  It  is  pointed  out  that  phosphorus  pentoxide  is  not  perceptibly  attacked 
by  hydrobromic-acid  gas  which  is  diluted  with  twice  its  volume  of  nitrogen. 


III. 

A   REVISION   OF   THE   ATOMIC  WEIGHT   OF 
MANGANESE. 

THE  ANALYSES  OF  MANGANOUS  BROMIDE  AND 

CHLORIDE. 


By  Gregory  Paul  Baxter  and  Murray  Arnold  Hines. 


Journal  of  the  American  Chemical  Society,  28,  1560  (1906). 

Zeitschrift  fvir  anorganische  Chemie,  51,  202  (1906). 

Chemical  News,  95,  102,  iii,  123  (1907). 

Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF  MANGANESE. 
THE   ANALYSES   OF  MANGANOUS   BROMIDE   AND   CHLORIDE. 


INTRODUCTION. 


The  following  table,  adapted  from  Clarke's  "A  Recalculation  of  the  Atomic 
Weights"  ^  gives  a  brief  resimie  of  previous  work  upon  the  atomic  weight  of 
manganese  which  has  other  than  historical  interest. 


Date. 

Investigator. 

Reference. 

Ratio  determined. 

Result. 

1830 

Berzelius    .    .    . 

Ann.  Physik.  Chem.,  18,  74 

MnCU:  2AgCl 

S5-IO 

1831 

Turner    .... 

Trans.  Roy.  Soc,  Edinb.  11, 143 

MnCla:  aAgCl 

54-90 

1857 

von  Hauer      .    . 

J.  pr.  Chem.,  72,  360 

MnS04:  MnS 

54-91 

1859 

Schneider   .    .    . 

Ann.  Physik.  Chem.,  107,  605 

Mn:  2CO2 

54-03 

Rawack  .... 

Ibid. 

MngO^iHaO 

54.08 

i860 

Dumas    .... 

Ann.  Chem.  Pharm.,  113,  25 

MnCU:  2Ag 

54-96 

1883 

Dewar  and  Scott 

Proc.  Roy.  Soc,  35.  44 

AgMn04:  AgMnO 
AgMn04:  KBr 

55-01 
5S-04 

i88s 

Marignac    .    .    . 

Arch.  sci.  phys.  nat.,  [3]  10,  21 

MnO:MnS04 

55-01 

1890 

Weeren   .... 

Dissertation,  Halle 

MnO:  MnSOi 
MnS:MnS04 

55-00 
55.00 

The  close  agreement  of  the  greater  part  of  these  determinations  is  striking, 
the  experiments  of  Schnieder  and  Rawack  being  the  only  ones  which  indicate  a 
value  for  manganese  very  different  from  55.0.  The  variations  of  their  results 
from  the  others  is  not  surprising,  however,  since  manganoso-manganic  oxide 
and  manganous  oxalate,  with  which  they  worked,  are  undoubtedly  difi&cult  to 
obtain  in  a  pure  condition.  The  remaining  determinations  all  fall  within  limits 
two  tenths  of  a  unit  apart,  and  all  but  two  agree  within  thirteen  hundredths  of 
a  unit. 

For  this  investigation  the  substances  chosen  for  examination  were  manganous 
bromide  and  chloride,  since  the  analysis  of  halogen  compounds  may  be  ef- 
fected with  great  accuracy.  Furthermore,  these  compounds  have  not  been  in- 
vestigated by  any  of  the  more  recent  experimenters  except  Dewar  and  Scott,^ 
who  performed  one  analysis  each  of  the  chloride  and  bromide  and  obtained  the 
values  54.89  and  54.95  respectively. 

»  Smith.  Misc.  Coll.,  The  Constants  of  Nature,  Part  V.,  1910.  The  results  have  been 
calculated  with  the  use  of  the  following  atomic  weights:  0  =  16.00;  C  =  12.00;  S  =  32.07; 
CL=  35-46;  K  =  39.10:  Ag  =  107.88. 

'  Loc.  cit. 

33 


34  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

THE   ANALYSIS    OF   MANGANOUS   BROMIDE. 
PURIFICATION   OF   MATERIALS. 

All  of  the  water  used  in  either  the  purification  or  the  analyses  was  twice  dis- 
tilled, once  from  a  dilute  alkaline  solution  of  potassium  permanganate  and  then 
from  a  very  dilute  sulphuric-acid  solution.  Block-tin  condensers  were  used  in 
both  distillations  and  the  apparatus  contained  no  rubber  or  cork  connections. 
The  water  was  collected  as  a  rule  in  Jena  glass  flasks,  although  for  special  pur- 
poses either  platinum  or  quartz  receivers  were  substituted. 

Acids  and  ammonia  also  were  distilled  shortly  before  use,  either  platinum  or 
quartz  condensers  and  receivers  being  employed  when  necessary.  Solid  reagents 
were  recrystallized,  usually  with  centrifugal  drainage. 

Special  pains  were  taken  in  all  the  work  to  prevent  the  introduction  of  alka- 
lies or  silica  into  the  purest  materials,  by  avoiding  as  far  as  possible  the  use  of 
glass  vessels. 

MANGANOUS  BROMIDE. 

Four  different  specimens  of  manganous  bromide  were  employed,  which  were 
obtained  from  different  sources  and  were  purified  in  different  ways.  In  the 
case  of  Samples  A  and  B,  purification  of  the  manganese  from  other  heavy  metals 
was  accompUshed  by  recrystallization  of  Merck's  "chemically  pure"  potassium 
permanganate.  Sample  A  was  crystallized  three  times  only,  while  Sample  B 
was  thus  treated  ten  times,  the  last  two  crops  of  crystals  being  thoroughly 
freed  from  the  mother-liquors  by  centrifugal  drainage. 

In  order  to  free  the  manganese  from  potassium  and  convert  it  into  the  bro- 
mide, the  following  processes  were  employed  with  Sample  A:  First  the  perman- 
ganate was  dissolved  in  water  and  was  reduced  by  passing  sulphur  dioxide  into 
the  solution.  This  sulphur  dioxide  was  made  by  heating  copper  turnings  with 
concentrated  sulphuric  acid  and  was  purified  from  copper  compoimds  mechani- 
cally carried  along  by  passing  through  three  gas  washing-bottles,  each  con- 
taining a  solution  of  sulphurous  acid,  and  one  column  of  beads  moistened  with 
a  similar  solution.  From  the  solution  of  potassium  and  manganous  sulphates 
the  manganese  was  precipitated  by  the  addition  of  an  alkaline  solution  of  am- 
monium carbonate.  The  manganous  carbonate  was  washed  with  water  imtil 
the  washings  were  free  from  sulphates,  then  it  was  dissolved  in  nitric  acid, 
which  had  been  redistilled  until  free  from  chlorine,  and  the  manganous  nitrate 
was  recrystallized  six  times  from  a  solution  strongly  acid  with  nitric  acid,  four 
times  in  a  glass  vessel,  twice  in  platinum. 

Usually  it  was  necessary  to  start  crystallization  by  inoculation,  and  cooling 
with  ice  was  found  advisable  for  the  sake  of  economy  in  material.  From  a  dilute 
solution  of  the  purified  nitrate  in  a  platinum  vessel,  the  manganese  was  again 
precipitated  as  carbonate,  by  means  of  ammonium  carbonate  which  had  been 
freshly  made  by  passing  pure  carbon  dioxide  into  distilled  ammonia  in  a  plati- 


A  REVISION  OF   THE  ATOMIC  WEIGHT   OF  MANGANESE.  3$ 

num  flask.  The  resulting  manganous  carbonate,  after  thorough  washing  with 
water  containing  a  small  amount  of  ammonia  to  prevent  colloidal  solution  of 
the  carbonate,  was  readily  converted  into  bromide  by  solution  in  hydrobromic 
acid.  Since  it  was  probable  that  the  carbonate  contained  occluded  nitrate,  and 
since  a  portion  of  the  material  had  been  oxidized  to  the  manganic  state  during 
the  washing,  it  was  obvious  that  bromine  would  be  set  free  during  the  solution 
in  hydrobromic  acid.  The  use  of  a  platimun  vessel  for  this  purpose  was  there- 
fore precluded.  In  order  to  avoid  the  introduction  of  silica,  fused  quartz 
dishes  were  employed,  instead  of  glass  vessels.  The  former  have  been  shown 
to  be  practically  insoluble  in  acid  solutions.^ 

The  free  bromine  was  expelled  from  the  solution  of  manganous  bromide  by 
prolonged  heating  on  a  steam-bath  in  a  quartz  dish.  Finally  it  was  crystallized 
six  times,  thrice  in  quartz,  and,  after  filtration  with  a  platinum  funnel,  thrice  in 
platinum  with  centrifugal  drainage  after  each  crystallization.  The  crystals 
were  dried  as  far  as  possible  over  stick  potash  in  a  vacuum  desiccator.  From 
the  mother-liquors,  by  means  of  six  similar  crystallizations.  Sample  A2  was 
obtained. 

In  the  conversion  of  Sample  B  from  permanganate  to  bromide  minor  changes 
were  introduced.  The  ammonium  carbonate  was  prepared  in  a  pure  state  by 
distilling  a  solution  of  commercial  ammonium  carbonate  in  a  platinum  still. 
Instead  of  expelling  free  bromine  from  the  solution  of  manganous  bromide  by 
prolonged  heating  upon  the  steam-bath,  the  solution  was  evaporated  as  far  as 
possible  upon  the  steam-bath  and  the  residue  was  heated  to  200°  in  an  electric 
oven.  The  bromide  was  dissolved  in  water,  and  after  filtration  of  the  solution, 
was  crystallized  three  times  in  a  platinum  dish.  The  third  crop  of  crystals  is 
designated  as  Sample  B. 

Sample  C  was  prepared  from  a  commercial  specimen  of  pyrolusite.  This  was 
first  dissolved  in  hydrochloric  acid  and  the  solution  was  boiled  to  expel  chlorine. 
Hydrogen  sulphide  was  passed  into  the  diluted  solution  of  manganous  chloride 
to  saturation,  and  the  precipitate  of  sulphur  and  sulphides  was  removed  by  fil- 
tration. After  the  excess  of  hydrogen  sulphide  had  been  expelled  by  boiling,  the 
solution  was  fractionally  precipitated  with  sodium  hydroxide  until  the  precipi- 
tate was  free  from  iron.  Finally,  the  manganese  was  precipitated  with  ammo- 
nium carbonate  and  the  precipitate  was  washed  and  dissolved  in  nitric  acid. 
The  nitrate  was  recrystallized  and  converted  into  bromide  exactly  as  in  the  case 
of  Sample  A. 

The  source  of  Sample  D  was  Merck's  "chemically  pure"  manganous  sul- 
phate. A  solution  of  500  gm.  of  this  salt  was  first  saturated  with  hydrogen  sul- 
phide, and  the  precipitate,  which  consisted  chiefly  of  manganous  sulphide,  was 
removed  by  filtration.  After  the  addition  of  a  small  amount  of  ammonia,  hy- 
drogen sulphide  was  again  passed  into  the  solution  to  saturation,  and  the  pre- 

»  Mylius  and  Meusser:  Zeit.  anorg.  Chem.,  44, 221  (1905).   (See  also  page  36  of  this  paper.) 


36  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

cipitate  discarded.  In  a  similar  way  third  and  fourth  fractions  of  sulphide  were 
removed.  Next  the  solution  was  thrice  fractionated  with  small  portions  of 
potassium  hydroxide,  the  precipitate  being  rejected  in  each  case.  Then  the 
manganese  was  twice  precipitated  as  carbonate  by  means  of  ammonium  car- 
bonate and  the  manganous  carbonate  was  converted  into  bromide  exactly  as  in 
the  case  of  Sample  B.  The  first  crop  of  thrice  recrystallized  bromide  is  desig- 
nated Sample  Di,  a  second  similar  crop  obtained  from  the  mother-liquors  is 
Sample  D2. 

HYDROBROMIC  ACID. 

Commercial  bromine  was  freed  from  chlorine  by  twice  converting  the  bro- 
mine into  hydrobromic  acid  by  means  of  thoroughly  washed  hydrogen  sulphide 
and  water,  and  heating  the  hydrobromic  acid,  after  distillation,  with  recrystal- 
lized potassium  permanganate.  The  bromine  was  thus  twice  distilled  from  a 
bromide,  the  bromide  in  the  second  distillation  being  almost  free  from  chloride. 
Iodine  was  eliminated  by  boiUng  the  hydrobromic  acid  in  each  case  with  a  small 
quantity  of  permanganate  and  rejecting  the  bromine  set  free.  A  portion  of  the 
final  product,  when  converted  into  ammonium  bromide  by  means  of  ammonia, 
and  added  to  a  solution  of  3.46875  gm.  (in  vacuimi)  of  pure  silver,  yielded 
6.03855  gm.  (in  vacuum)  of  fused  silver  bromide,  whence  the  ratio  of  silver  to 
silver  bromide  is  57.443,  while  57.445  is  the  value  to  be  expected.^ 

By  treating  this  bromine,  covered  with  water,  with  washed  hydrogen  sulphide, 
hydrobromic  acid  was  again  produced.  The  solution  was  boiled,  after  mechani- 
cal separation  of  the  greater  part  of  the  free  sulphur  and  bromide  of  sulphur, 
and  was  then  filtered.  In  order  to  remove  the  sulphuric  acid  produced  during 
the  action  of  the  bromine  upon  the  hydrogen  sulphide,  the  hydrobromic  acid 
was  first  distilled.  Then  it  was  diluted,  and  a  small  quantity  of  recrystallized 
barium  hydroxide  was  added  to  precipitate  last  traces  of  sulphuric  acid.  The 
slight  precipitate  of  bariimi  sulphate  was  collected  upon  a  filter,  and  the  acid 
was  three  times  distilled  with  rejection  of  the  first  and  last  portions,  with  a 
glass  retort  and  condenser.  Finally  the  acid  was  once  distilled  with  the  use  of  a 
quartz  condenser.  The  product  of  the  final  distillation  was  collected  in  quartz 
vessels  and  was  used  immediately  for  dissolving  the  manganous  carbonate.  That 
this  acid  was  free  from  solid  impurities,  such  as  alkalies  and  silica,  was  shown 
by  evaporating  30  c.c.  in  a  weighed  platimmi  crucible.  No  weighable  residue 
remained  after  the  crucible  had  been  heated  to  very  dull  redness. 

NITRIC  ACID. 

This  acid  was  twice  distilled,  all  but  the  last  third  of  the  distillate  being 
rejected  in  each  distillation.  This  acid  gave  no  test  for  chloride  in  a  nephel- 
ometer. 

*  Baxter:  Proc.  Amer.  Acad.,  ^2,  210  (1906);  Jour.  Amer.  Chem.  Soc,  28,  1332;  Zeit. 
anorg.  Chem.,  50, 398.     (See  page  59.) 


A  REVISION  OF   THE  ATOMIC  WEIGHT   OF   MANGANESE.  37 

SILVER. 

Five  different  specimens  of  silver  were  employed,  a  portion  of  each  one  of 
which  had  already  been  used  in  an  atomic  weight  research,  and  had  been  shown 
to  be  of  the  highest  grade  of  purity.  Two  of  these  specimens.  Samples  H  and  J, 
were  used  in  an  investigation  upon  the  atomic  weight  of  iodine  by  one  of  us.^ 
Sample  H  was  prepared  from  silver  nitrate  which  had  been  seven  times  re- 
crystallized  from  nitric  acid,  five  times  recrystaUized  from  water,  and  finally 
precipitated  with  ammonium  formate.  Sample  J  was  precipitated  once  as  silver 
chloride,  electrolyzed  once,  and  finally  precipitated  with  ammonium  formate. 
Sample  K  was  used  in  our  first  investigation  upon  the  atomic  weight  of  cad- 
mium.2  This  sample  was  thrice  precipitated  as  silver  chloride  and  once  elec- 
trolyzed. Sample  L  was  precipitated  once  as  chloride,  once  as  metal  by  am- 
monium formate  and  was  once  electrolyzed.  This  sample  has  been  used  in  the 
analysis  of  cadmium  bromide.^  Sample  M  was  prepared  for  an  investigation 
upon  the  atomic  weight  of  bromine,  and  had  been  twice  electrolyzed  after  a 
preliminary  purification."*  Samples  H,  J,  and  L  also  were  used  in  the  latter  re- 
search, and  were  found  to  give  values  identical  with  those  obtained  with  Sample 
M.  All  five  samples  were  finally  fused  in  a  current  of  pure  hydrogen  in  a  lime 
boat.  The  fused  lumps  were  cleaned  with  dilute  nitric  acid,  cut  into  fragments 
either  with  a  clean  steel  chisel  and  anvil,  or  with  a  jeweler's  saw,  treated  with 
dilute  nitric  acid  until  free  from  iron,  washed,  dried,  and  finally  heated  to  about 
300°  in  a  vacuum. 

DRYING    OF   MANGANOUS   BROMIDE. 

The  method  of  analysis  was  essentially  that  usually  employed  in  this 
laboratory  for  the  analysis  of  metallic  halides,  and  has  been  described  in 
the  preceding  papers  on  the  cadmium  halides.  Weighed  portions  of  the 
bromide,  after  fusion  in  hydrobromic  acid,  were  first  titrated  against  weighed 
portions  of  pure  silver.  Then  the  precipitated  silver  salt  was  collected  and 
weighed. 

The  apparatus  used  for  the  fusion  of  the  manganous  bromide  in  a  current  of 
nitrogen  and  hydrobromic-acid  gases  has  already  been  described  in  detail  on 
page  23. 

The  manganous  bromide,  contained  in  a  weighed  platinum  boat,  was  heated 
gently  in  a  current  of  nitrogen,  until  the  greater  part  of  the  crystal  water  was 
expelled,  then  strongly  in  a  current  of  nitrogen  and  hydrobromic  acid  until 
fused.  After  the  salt  had  cooled,  the  hydrobromic  acid  was  displaced  by  nitro- 
gen and  this  in  turn  by  pure  dry  air,  the  purif)dng  apparatus  being  constructed 
in  such  a  way  that  by  means  of  stop-cocks  any  one  gas  or  mixture  of  gases  could 
be  employed,  to  the  exclusion  of  the  others.    The  boat  was  then  transferred  to 


^  See  page  108.  ^  See  page  6. 

'  Seepage  22.  *  See  page  56. 


38  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

the  weighing-bottle  in  which  it  was  originally  weighed,  and  the  stopper  was  in- 
serted without  an  instant's  exposure  of  the  salt  to  moisture,  by  means  of  the 
bottling  apparatus  described  on  page  9.  The  weighing-bottle  was  then  al- 
lowed to  stand  in  a  desiccator  near  the  balance  case  for  some  time  before  it  was 
weighed. 

METHOD   OF  ANALYSIS. 

Next  the  boat  was  transferred  to  a  flask  and  the  salt  was  dissolved  in  about 
300  c.c.  of  the  purest  water.  The  weighing-bottle  was  rinsed  and  the  rinsings 
were  added  to  the  solution.  Then  the  solution  was  filtered  into  the  glass  stop- 
pered precipitating  flask  through  a  tiny  filter  to  collect  a  trace  of  insoluble 
matter,  and  the  filter-paper  and  residue  were  ignited  in  a  weighed  porcelain 
crucible. 

From  the  weight  of  manganous  bromide  very  nearly  the  requisite  quantity 
of  pure  silver  could  be  calculated.  This  silver  was  weighed  out  and  after  solu- 
tion in  nitric  acid  as  described  on  page  12,  and  dilution  until  not  stronger  than 
I  per  cent,  the  solution  was  slowly  added  with  constant  stirring  to  the  i  per 
cent  solution  of  manganous  bromide  in  the  precipitating  flask.  After  having 
been  shaken  for  some  time,  the  solution  was  allowed  to  stand  several  days,  one 
week  in  the  case  of  analyses  14  and  15,  with  occasional  shaking,  imtil  the  super- 
natant liquid  was  clear.  30  c.c.  portions  of  the  solution  were  then  tested  with 
hundredth-normal  solutions  of  silver  nitrate  and  sodiimi  bromide  in  the  nephel- 
ometer  for  excess  of  bromide  or  silver,  and,  if  necessary,  either  standard  silver 
nitrate  or  sodium  bromide  solution  was  added,  and  the  process  of  shaking  and 
testing  repeated,  until  the  amounts  of  bromide  and  silver  in  the  solution  were 
equivalent.  If  the  solution  was  perfectly  clear  when  tested,  and  contained  no 
considerable  excess  of  bromide  or  silver,  the  test  solutions  were  discarded,  since 
they  contained  only  negligible  amounts  of  silver  bromide,  otherwise  they  were 
returned  to  the  flask  and  a  correction  was  applied  for  the  silver  bromide  thus 
introduced. 

As  soon  as  the  exact  end-point  of  the  titration  had  been  found,  about  4  eg. 
of  silver  nitrate  in  excess  were  added  to  precipitate  dissolved  silver  bromide 
and  the  solution  was  again  shaken  and  allowed  to  stand  until  clear.  The  pre- 
cipitate of  silver  bromide  was  collected  upon  a  weighed  Gooch  crucible  after  it 
had  been  washed  with  water  by  decantation  about  ten  times.  Next  it  was  heated 
for  several  hours  at  140°,  then  for  2  hours  at  about  230°  in  an  electric  air-bath, 
and,  after  it  had  cooled  in  a  desiccator,  it  was  weighed.  In  order  to  determine 
how  much  moisture  was  retained  by  the  precipitate,  in  each  case  it  was  trans- 
ferred as  completely  as  possible  to  a  clean  porcelain  crucible  and  weighed;  then 
the  salt  was  fused  by  heating  the  small  crucible,  contained  in  a  large  covered 
crucible,  and  again  weighed.  The  fused  salt  was  light  yellow  as  a  rule,  showing 
that  no  appreciable  reduction  had  taken  place.  The  asbestos  mechanically  de- 
tached from  the  Gooch  crucible  together  with  a  minute  quantity  of  silver  bro- 


A  REVISION  OF   THE  ATOMIC  WEIGHT  OF  MANGANESE.  39 

mide  which  occasionally  escaped  the  crucible,  was  collected  from  the  filtrate 
and  wash-waters  upon  a  small  filter,  the  ash  of  which  was  treated  with  nitric 
and  hydrobromic  acids  before  weighing.  Although  the  filtrates  and  first  wash- 
waters  were  essentially  free  from  dissolved  silver  bromide,  the  subsequent  wash- 
waters  usually  contained  a  trace  of  silver  bromide.  The  amount  of  dissolved 
salt  was  determined  with  the  nephelometer  by  comparison  with  standard,  bro- 
mide solutions. 

Several  difliculties  were  met  in  carrying  out  the  analyses.  In  the  first  place  it 
proved  difficult  to  wash  the  platinum  boat  absolutely  clean.  When  rinsed  with 
cold  water  only,  and  dried  at  ioo°,  the  weight  was  in  many  cases  a  few  hun- 
dredths of  a  milligram  greater  than  before  fusion  of  the  bromide.  Ignition  to 
redness  of  the  boat  thus  treated  then  produced  a  slight  loss  in  weight.  Rinsing 
with  hot  water  reduced  the  gain  in  weight  of  the  boat  after  drying  but  did  not 
wholly  prevent  a  slight  loss  on  ignition.  The  cause  of  the  variation  was  not 
discovered,  hence  it  seems  safer  in  the  calculations  to  use  the  weight  of  the  boat 
after  drying.  The  total  variation  is  so  slight,  however,  that  it  scarcely  affects 
the  final  result. 

Two  other  difficulties  arose  from  the  fact  that  when  a  solution  of  a  manganous 
salt,  even  as  dilute  as  the  filtrates  from  analyses,  is  filtered  through  filter-paper, 
in  spite  of  long-continued  washing  a  small  amount  of  manganese  is  tenaciously 
retained  by  the  paper.  This  was  discovered  from  the  fact  that  the  asbestos 
residues  always  contained  manganese.  In  analyses  29  to  31  it  was  found 
possible  to  eliminate  the  manganese  completely  by  washing  the  filter  finally  with 
5  per  cent  hydrobromic  acid.  In  two  cases  the  residues  were  analyzed  for 
manganese  and  were  found  to  contain  0.00023  3-nd  0.00057  S^-  o^  ^1^3  O4 
respectively. 

The  average  of  these  two  quantities  is,  however,  larger  than  the  total  residue 
in  some  cases,  hence  this  value  can  not  be  used  to  correct  the  previous  analyses. 
In  order  to  determine  accurately  the  proper  correction  for  this  error,  a  solution 
containing  manganous  nitrate  in  the  porportion  in  which  it  was  usually  con- 
tained in  the  filtrate  of  an  analysis  was  passed  through  filter-papers  and  the  fil- 
ters were  then  washed  as  thoroughly  as  possible  with  water.  The  ash  of  these 
papers  invariably  contained  manganese,  the  weights  of  manganic  oxide  in  sev- 
eral experiments  being  found  to  be  0.00018,  o.oooii,  0.00006,  0.00018  and 
0.00005  S^-  with  an  average  of  0.00012  gm.  This  quantity  was  subtracted 
from  the  weight  of  asbestos  shreds  in  all  cases  except  analyses  29  to  31,  where 
the  paper  was  washed  with  hydrobromic  acid. 

The  residue  obtained  by  the  filtration  of  the  manganous  bromide  proved  to 
contain  manganese  and  to  be  free  from  detectable  amounts  of  platinum  and 
silica.  Probably  this  insoluble  residue  consisted  chiefly  of  oxides  of  mangan- 
ese, although  prolonged  fusion  in  hydrobromic  acid  failed  to  reduce  materially 
the  proportion  of  insoluble  matter.  The  discovery  of  adsorption  in  the  case 
of  manganous  nitrate  led  to  the  suspicion  that  at  least  a  portion  of  this  residue 


40 


RESEARCHES  UPON   ATOMIC  WEIGHTS. 


was  due  to  adsorption  of  manganese  compounds  by  the  filter-paper.  In  order 
to  test  this  point  a  solution  of  manganous  bromide,  containing  about  5  gm. 
in  200  c.c,  after  one  filtration  was  again  filtered  through  a  second  filter  about 
3  cm.  in  diameter.  Filters  of  this  size  were  used  in  filtering  the  manganous 
bromide  solution  in  the  analyses.  This  filter  was  then  washed  with  water  as  thor- 
oughly as  in  an  analysis,  and  was  ignited.  The  weight  of  manganic  oxide  ob- 
tained was  0.00008  gm.  Two  repetitions  of  the  experiment  pelded  0.00008  and 
o.oooio  gm.  respectively.  If,  as  is  probable,  the  manganese  is  adsorbed,  not  as 
bromide,  but  as  some  basic  compound,  possibly  as  manganic  hydroxide,  the  bro- 
mine would  have  remained  partially,  if  not  wholly,  in  the  solution.  In  that  case 
a  suitable  correction  could  be  applied  by  subtracting  from  the  weight  of  the 
residue  the  average  of  the  quantities  of  manganese  adsorbed  in  the  above  exper- 
iments. An  attempt  to  prevent  the  difficulty  by  adding  dilute  sulphuric  acid 
to  the  solution  of  manganous  bromide  did  not  diminish  the  extent  of  the  adsorp- 
tion. Hence  a  negative  correction  of  0.00009  S^-  is  applied  to  the  residue  in  all 
cases. 

It  has  already  been  shown  that  chlorides  and  bromides  which  have  been 
fused  and  allowed  to  solidify  in  an  acid  atmosphere  occlude  none  of  the  gas,  for 
they  give  neutral  solutions.^ 


THE   DENSITY   OF   MANGANOUS   BROMIDE. 

In  order  to  find  accurately  the  vacuum  correction  for  manganous  bromide, 
it  was  necessary  to  determine  the  density  of  this  salt.  The  experiments  were 
carried  out  exactly  as  described  in  our  determinations  of  the  specific  gravities 
of  cadmium  halides,^  with  the  following  results: 

Density  of  MnBr2. 
Density  of  Toluol  25°/  4°  =  0.86156. 


Weight  of  MnBr2 
in  vacuum. 

Weight  of  toluol 
displaced  in  vacuum. 

Density  of  MnBrj. 

2S°/4° 

gm. 

3.0098 
3.0342 

gm. 
0.5914 
0.5963 

4.38s 
4-384 

Aver.  4.385 

The  average  density  of  several  of  the  brass  weights  was  found  by  displace- 
ment of  water  to  be  8.3. 

1  Richards:  Proc.  Amer.  Acad.,  29,  59  (1893);  Zeit.  anorg.  Chem.,  6,  93  (1894);  Jour. 
Amer.  Chem.  Soc,  24,  376  (1902);  Zeit.  anorg.  Chem.,  31,  273;  Richards  and  Baxter:  Proc. 
Amer.  Acad.,  34,  367  (1899);  Zeit.  anorg.  Chem.,  21,  269;  Baxter  and  Hines:  Jour.  Amer. 
Chem.  Soc,  27,  227  (1905);   Zeit.  anorg.  Chem.,  44,  163. 

'  Amer.  Chem.  Jour.,  31,  220  (1904). 


A  REVISION  OF   THE  ATOMIC  WEIGHT  OF   MANGANESE. 


41 


All  weights  were  reduced  to  the  vacuum  standard  by  applying  the  following 
corrections  for  each  apparent  gram  of  substance. 


Specific  gravity. 

Vacuum  correction. 

Weights 

MnBra 

AgBr 

Ag 

Toluol 

8.3 
4.385 
6.473 
10.50 
0.862 

+0.000129 
4-0.00004I 
—0.000031 
+0.00126 

The  balance  was  a  new  Troemner,  No.  10,  and  was  easily  sensitive  to  0.02 
mg.  with  a  load  of  less  than  50  gm.  The  weights,  which  were  of  brass,  gold- 
plated,  were  occasionally  carefully  standardized  to  hundredths  of  a  milligram. 
The  corrections  did  not  vary  with  time,  however.  All  weighings  were  made  by 
substitution,  with  tare  vessels  as  nearly  like  those  being  weighed  as  possible. 
(See  tables  on  pp.  42  and  43.) 

The  close  agreement  of  the  averages  of  the  two  series  is  conclusive  evidence 
that  no  serious  error,  such  as  occlusion  by  the  silver  bromide,  affected  the 
method  of  analysis.  This  is  strikingly  shown  by  the  ratio  between  the  silver 
used  and  the  silver  bromide  obtained  in  the  same  analysis. 

Ag  :  AgBr 
Analyses     i  and  18 57.4438 

2  "  19 57-4459 

3  "  20 57.4426 

4  "  21 57.4425 

5  "  23 57-4439 

8  "  24 57-4439 

9  "  25 57-4457 

10  "  26 .  57-4432 

11  "  27 57-4423 

12  "  28 57.4421 

13  "  29 57-4454 

14  "  30 57.4387 

15  "31 57-4456 

Average,    57-4435 
Average,  rejecting  analyses  14  and  30   .   .   .    57.4439 

The  most  probable  value  for  this  ratio  has  recently  been  shown  to  be  57.4453-^ 
Although  the  foregoing  figures  furnish  strong  e\ddence  that  the  atomic 
weight  of  manganese  lies  very  close  to  54.93,  it  seemed  advisable  to  attempt  to 
confiirm  this  result  by  the  analysis  of  other  manganese  compounds.  The  suc- 
cess which  accompanied  this  investigation  of  the  bromide  led  to  the  selection  of 
manganous  chloride  for  the  next  series  of  experiments.  A  comparison  of  the 
different  specimens  of  material  and  a  discussion  of  all  the  results  is  to  be  found 
at  the  end  of  this  paper. 

*  Baxter:  Proc.  Amer.  Acad.,  ^2,  210  (1906);  Jour.  Amer.  Chem.  Soc,  28,  1332;  Zeit. 
anorg.  Chem.  50,  398.    (See  page  59.) 


42 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


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A  REVISION   OF   THE   ATOMIC  WEIGHT   OF   MANGANESE. 


43 


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44  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

ANALYSIS   OF   MANGANOUS   CHLORIDE. 

PURIFICATION    OF    MATERIALS. 
MANGANOUS  CHLORIDE. 

Portions  of  the  pure  material  employed  in  the  experiments  upon  manganous 
bromide  served  also  for  the  preparation  of  manganous  chloride.  Two  speci- 
mens of  chloride  were  made.  Sample  B  corresponds  in  its  purity  to  Sample  B 
of  manganous  bromide,  since  both  were  made  by  dissolving  the  same  specimen  of 
manganous  carbonate  (page  35).  This  carbonate  had  been  made  from  potas- 
sium permanganate  which  had  been  crystallized  ten  times.  The  solution  of  the 
carbonate  in  hydrochloric  acid  was  evaporated  to  dryness  in  a  quartz  dish  and 
was  heated  to  200°  in  an  electric  oven.  Then  the  filtered  aqueous  solution  was 
evaporated  to  crystallization  in  a  platinum  dish  and  the  first  crop  of  crystals 
was  twice  recrystallized  (Sample  B). 

The  mother-liquors  of  the  manganous  nitrate  used  in  preparing  Samples  B 
and  D  of  bromide  (page  35)  were  combined  and  crystallized  six  times,  three 
times  in  glass  vessels,  three  times  in  platinum  vessels,  and  from  this  pure  ni- 
trate Sample  E  was  prepared  by  precipitation  of  the  carbonate  with  distilled 
ammonium  carbonate,  solution  in  hydrochloric  acid  and  crystallization  as  with 
Sample  B. 

HYDROCHLORIC  ACID. 

The  hydrochloric  acid  was  purified  by  boiling  the  "chemically  pure"  acid  for 
some  time,  after  the  addition  of  a  small  quantity  of  potassium  permanganate. 
Then  it  was  distilled  in  glass,  the  middle  portion  only  being  collected,  and  finally 
once  with  a  quartz  condenser  shortly  before  use. 

SILVER. 

The  same  samples  of  pure  silver  were  employed  as  in  the  work  with  mangan- 
ous bromide  (page  37). 

DRYING   OF   MANGANOUS   CHLORIDE. 

The  salt  was  prepared  for  analysis  by  fusion  in  a  current  of  hydrochloric-acid 
gas  and  then  the  chlorine  content  was  determined  gravimetrically  as  silver 
chloride  and  by  titration  against  a  weighed  amount  of  silver  in  solution. 

The  apparatus  employed  for  the  fusion  was  identical  with  that  described  in 
the  paper  on  cadmium  chloride  (see  page  8),  the  hydrochloric-acid  gas  in 
which  the  fusion  took  place  being  dried  by  passing  through  four  towers  con- 
taining concentrated  sulphuric  acid.  In  a  few  preliminary  experiments  the 
hydrochloric-acid  gas  was  passed  over  freshly  sublimed  phosphorus  pentoxide, 
but  this  drying  agent  was  subsequently  eliminated  since  it  was  found  to  intro- 
duce volatile  phosphorus  compounds  into  the  hydrochloric-acid  gas,  and  thus 
into  the  manganous  chloride,  if  the  latter  was  moist.    This  difficulty  has  been 


A  REVISION   OF   THE   ATOMIC   WEIGHT   OF   MANGANESE.  45 

discussed  elsewhere  (page  14),  and  in  the  same  place  it  has  been  shown  that 
hydrochloric-acid  gas  attacks  concentrated  sulphuric  acid  to  so  slight  an  ex- 
tent that  the  latter  may  safely  be  used  to  dry  the  former. 

THE   METHOD   OF   ANALYSIS. 

The  principal  point  of  difference  between  the  analyses  of  manganous  bro- 
mide and  manganous  chloride  was  occasioned  by  the  greater  solubility  of  silver 
chloride.  This  introduced  no  difficulty  in  the  case  of  the  titrations,  although 
the  opalescence  in  both  nephelometer  tubes  was  of  course  more  marked  at  the 
end-point.  In  order  to  make  the  precipitation  of  the  silver  chloride  more 
complete,  however,  a  much  larger  excess  of  silver  nitrate  was  necessary  than  in 
the  case  of  silver  bromide,  about  0.15  gm.  being  added  in  each  analysis.  Fur- 
thermore, the  precipitate  was  washed  at  first  with  a  solution  of  silver  nitrate 
containing  0.04  gm.  in  a  liter,  although  the  final  six  washings  were  performed 
with  water.  The  silver  chloride  dissolved  in  both  filtrate  and  washings  was  de- 
termined by  comparison  with  standard  solutions  in  a  nephelometer  as  previously 
described.    Here  even  the  filtrate  showed  a  trace  of  dissolved  silver  chloride. 

In  the  case  of  manganous  chloride  also  the  extent  of  the  adsorption  of  man- 
ganese compounds  by  the  filter-paper  was  investigated  by  filtering  a  solution 
of  the  salt  through  a  filter-paper,  and,  after  thoroughly  washing  the  paper,  de- 
termining the  residue  after  ignition.  The  correction  for  adsorption  thus  found, 
0.00006  gm.,  is  in  some  cases  larger  than  the  weight  of  the  residue  minus  the  loss 
in  weight  of  the  boat,  owing  possibly  to  the  fact  that  not  all  the  platinum  lost 
by  the  boat  is  collected  upon  the  filter-paper.  At  any  rate  the  uncertainty  in- 
troduced by  applying  the  correction  wherever  possible  is  very  small. 

The  filter  upon  which  the  asbestos  shreds  were  collected  was  washed  with 
warm  dilute  hydrochloric  acid  to  eliminate  adsorbed  manganese  compounds,  so 
that  no  correction  for  adsorption  is  here  necessary.  This  acid  was  then  tested 
for  silver  chloride  with  the  nephelometer,  and  if  any  was  found  it  was  added  to 
the  main  weight  of  this  substance. 

In  analyses  i,  2,  3,  4,  and  7  the  silver  nitrate  was  added  to  the  manganous 
chloride,  while  in  analyses  5  and  6  precipitation  was  performed  in  the  reverse 
fashion. 

The  possibility  of  the  existence  of  manganic  compounds  in  the  salt  was  con- 
sidered. In  order  to  determine  whether  or  not  this  was  the  case,  one  specimen 
of  manganous  chloride,  after  fusion  in  hydrochloric  acid,  was  tested  for  man- 
ganic compounds  by  adding  a  solution  of  starch  paste  and  potassium  iodide. 
No  coloration  resulted  even  after  adding  hydrochloric  acid,  although  a  mere 
trace  of  permanganate  produced  immediate  coloration  in  the  same  solution. 

The  fused  bromide  and  chloride  were  both  pink,  hence  Forchhammer's  state- 
ment that  the  pink  color  of  manganous  salt  is  due  to  the  presence  of  manganic 
compounds  is  incorrect.* 

*  Ann.  Phil,  N.  S.,  i,  50  (1821). 


46 


RESEARCHES  UPON  ATOMIC  WEIGHTS. 


DENSITY   OF   MANGANOUS   CHLORIDE. 


In  order  to  find  the  vacuum  correction  for  manganous  chloride  the  density  of 
the  salt  was  determined  by  displacement  of  toluol.  This  was  done  with  the 
special  pycnometer  which  was  devised  for  the  determination  of  the  densities  of 
hygroscopic  salts,^ 


Density  of  MnCU. 
Density  of  toluol  2s°/4°  =  0.86166. 

Weight  of  MnClj 
in  vacuum. 

Weight  of  toluol 
displaced  in  vacuum. 

Density  of  MnCU. 

25°/4°. 

gm. 
1.9436 
2.8532 
3.1202 

gm,. 
0.5617 
0.8266 

0.9035 

2.982 
2.974 
2.976 

Average,      2.977- 

Weighings  were  made  with  the  precautions  usual  in  exact  work.  All  weights 
were  reduced  to  the  vacuum  standard  by  applying  the  following  corrections 
for  each  apparent  gram  of  substance: 


Specific  gravity. 

Vacuum  correction. 

Weights 

MnCla 
AgCl 
Ag 
Toluol 

8.3 
2.977 
5.56 
10.49 
0.862 

+0.000259 
+0.000071 
—0.000031 
+0.00126 

RESULTS   AND  DISCUSSION. 

Richards  and  Wells  have  proved  beyond  question  that  the  ratio  of  silver  to 
silver  chloride  is  as  low  as  75.2634/  a  value  which  is  essentially  identical  with 
that  obtained  from  the  weights  of  silver  used  and  the  weights  of  silver 
chloride  obtained.  Ag  :  Agci. 

Analyses   i  and  8 75.2615 

2  "  9 75.2662 

3  "  10 75.2628 

4  "  II 75-2607 

5  "  12 75-2595 

6  "  13 75.2662 

7  "  14 75.2656 

Average,     75.2632 

Not  only  are  the  values  obtained  in  the  two  series  of  analyses  identical,  but 
they  differ  only  by  an  exceedingly  small  amount  from  those  resulting  from 
the  analysis  of  manganous  bromide  (pages  42  and  43). 

^  Baxter  and  Hines:  Anier.  Chem.  Jour.,  31,  220  (1904). 

2  Schroder  obtained  the  value  2.478.    Dichtigkeitsmessungen.  Heidelberg,  1873. 
^  Ptib.  Car.  Inst.,  No.  28,  65  (1905);  Jour.  Amer.  Chem.  Soc,  27,  520;  Zeit.  anorg.  Chem., 
47, 125- 


A  REVISION   OF   THE   ATOMIC  WEIGHT  OF  MANGANESE.  47 

The  final  averages  of  the  four  series  are  given  in  the  following  table: 

Series      I.    MnBr2: 2kg     54-934 

II.    MnBr2: 2AgBr 54-93° 

III.  MnCl2:  2Ag 54-933 

IV.  MnClz:  zAgCl 54-933 


Average,        54-933 

It  is  interesting  to  tabulate  the  analyses  according  to  the  specimens  of  mate- 
rial employed. 

Averages  Averages 

Samples  A  of  MnBra 54-934  Sample  H  of  Ag 54-933 

B  of  MnBr2 54-933  J  of  Ag 54-936 

C  of  MnBr2 54-933  K  of  Ag 54-934 

D  of  MnBr2     ....    54.926^  L  of  Ag 54-934 

B  of  MnCl2 54-935  M  of  Ag      54-933 

^°f^^C^^ 5_4^93i^  General  average,        1^:^ 

General  average,       54-932 

The  purification  of  Sample  B  was  exceptionally  thorough.  In  the  first  place 
potassium  permanganate  is  isomorphous  with  a  comparatively  limited  number 
of  substances,  so  that  the  initial  purification  by  a  large  number  of  crystallizations 
of  this  substance  may  be  expected  to  have  eliminated  every  trace  of  impurity 
of  heavy  metals.  In  the  second  place  the  final  product  had  been  many  times 
recrystallized  in  the  form  of  two  other  compounds  of  manganese.  It  is  almost 
inconceivable  that  any  impurity  could  have  eluded  this  large  number  of  crys- 
tallizations in  three  different  forms.  That  such  a  prolonged  purification  was 
imnecessary  is  shown  by  the  agreement  of  the  results  from  the  other  samples 
with  that  from  Sample  B.  Even  in  the  case  of  Samples  A,  C,  D,  and  E  the 
final  product  had  been  recrystallized  in  at  least  two  forms.  The  slightly  lower 
results  in  the  case  of  Sample  D  differ  from  the  others  by  an  amount  no  greater 
than  the  possible  experimental  error,  and  can  not  be  considered  to  indicate  that 
the  composition  of  this  sample  is  different  from  that  of  the  others.  The  vari- 
ous samples  of  silver  also  yielded  essentially  the  same  result,  which  confirms 
the  statement  as  to  the  identity  of  the  different  samples  made  on  page  37. 

There  can  be  little  doubt  that  the  final  average  of  all  four  series,  54.933,  repre- 
sents with  accuracy  the  relation  of  the  atomic  weight  of  manganese  to  that  of 
silver  107.880.  This  value  is  in  close  agreement  with  the  value  derived  from 
previous  investigations. 

The  main  results  of  this  research  may  be  briefly  summarized  as  follows: 

(i)  The  atomic  weight  of  manganese,  referred  to  silver  107.880,  is  found  to 
be  54.933,  by  analysis  of  both  manganous  bromide  and  manganous  chloride. 
With  silver  at  107.870,  manganese  becomes  54.927. 

(2)  The  specific  gravity  of  manganous  bromide  at  25°,  referred  to  water  at 
4°,  is  found  to  be  4.385,  and  that  of  manganous  chloride  under  the  same  con- 
ditions is  found  to  be  2.977. 

^  Analysis  30  is  rejected. 


48 


RESEARCHES   UPON  ATOMIC  WEIGHTS. 


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IV. 

A    REVISION    OF   THE   ATOMIC   WEIGHT   OF 

BROMINE. 

THE  SYNTHESIS  OF  SILVER  BROMIDE  AND  THE  RATIO 
OF  SILVER   BROMIDE   TO   SILVER   CHLORIDE. 


By  Gregory  Paul  Baxter. 


Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  42,  201  (1906). 

Journal  of  the  American  Chemical  Society,  28,  1322  (1906). 

Zeitschrift  fUr  anorganische  Chemie,  50,  389  (1906). 

Chemical  News,  94,  260,  261  (1906). 

Experimentelle  Untersuchungen  iiber  Atomgewichte,  page  768.    T.  W.  Richards.     1909. 

Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF  BROMINE. 

THE  SYNTHESIS  OF  SILVER  BROMIDE  AND  THE  RATIO  OF  SILVER 
BROMIDE   TO   SILVER  CHLORIDE. 


INTRODUCTION. 


In  numerous  investigations  in  this  laboratory  upon  the  atomic  weights  of 
certain  metals,  in  which  metallic  bromides  were  first  titrated  against  the  purest 
silver,  and  then  the  precipitated  silver  bromide  was  collected  and  weighed,  the 
relation  between  the  silver  used  in  the  titrations  and  the  silver  bromide  ob- 
tained has  yielded  data  from  which  the  atomic  weight  of  bromine  may  be  calcu- 
lated. Furthermore,  in  all  these  investigations,  as  a  check  upon  the  purity  of 
the  silver  and  bromine  employed,  silver  bromide  was  synthesized  directly  from 
weighed  quantities  of  silver  and  an  excess  of  ammonium  bromide  or  hydrobro- 
mic  acid.  Many  of  these  results  have  already  been  collected  and  discussed  by 
Richards,^  nevertheless  they  are  cited  in  the  following  table  together  with  a 
few  more  recent  determinations.     (See  table  on  page  52.) 

From  the  first  of  these  ratios  the  atomic  weight  of  bromine,  referred  to  silver 
107.880,  is  found  to  be  79.919,  and  from  the  second  79.918. 

Very  recently,  in  experiments  in  which  silver  iodide  was  heated  first  in  a  cur- 
rent of  air  and  bromine  until  the  iodine  was  completely  displaced,  and  then  in 
a  current  of  chlorine  to  displace  the  bromine,  the  ratio  of  silver  bromide  to 
silver  chloride  was  determined  in  six  cases.  From  the  results  of  these  experi- 
ments the  atomic  weight  of  bromine  was  calculated  to  be  79.916,^  if  the  atomic 
weight  of  chlorine  is  assumed  to  be  35.457. 

These  values  for  bromine  are  in  close  agreement  with  those  of  Stas.'  In  his 
experiments  weighed  quantities  of  pure  silver  and  bromine  were  first  titrated 
against  each  other,  and  then  the  precipitate  of  silver  bromide  was  collected  and 
weighed.  Of  the  four  results  by  the  first  method,  one  should  be  rejected  ac- 
cording to  his  own  statements,  since  the  bromine  was  not  thoroughly  dried. 
The  remaining  three,  79.922,  79.924,  and  79.923,  give  as  an  average  79.923. 
From  the  weight  of  silver  bromide  foiu:  values  were  obtained,  79.913,  79.9XSi 
79.918,  and  79.920,  with  an  average  of  79.917. 

*  Proc.  Amer.  Phil.  Soc,  43,  119  (1904). 

*  Baxter:  Proc.  Amer.  Acad.,  41, 82  (1905);  Jour.  Amer.  Chem.  Soc,  27,  884;  Zeit.  anor  g. 
Chem.,  46,  45.     (See  page  III.) 

*  (Euvres  Completes,  i,  603.  51 


52 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


Marignac  ^  also  determined  the  ratio  of  silver  to  silver  bromide,  with  some- 
what lower  results,  —  79.922,  79.904,  and  79.915;  average,  79.913. 

Scott,^  in  his  analyses  of  ammonium  bromide,  obtained  six  values  for  the 
same  ratio,  varying  between  79.899  and  79.911,  with  an  average  of  79.906. 
One  of  his  results  is  here  rejected,  since  the  silver  used  in  this  experiment  was 
known  to  be  impure. 

iNDrRECT  Determinations. 


Ratio 

No. 

Bromide 
analyzed. 

No.  of 
experiments. 

Analyst. 

Reference. 

Ag 
AgBr 

I 

BaBrz 

Last  seven 

Richards 

Proc.  Amer.  Acad.,  28,  28 

57-444 

2 

SrBr2 

Seven 

Richards 

Ibid.,  30,  389 

57-444 

3 

ZnBrz 

One 

Richards 

Ibid.,  31,  178 

57-445 

4 

NiBr2 

Seven 

Cushman 

Ibid.,  33,  III 

57-444 

5 

CoBr2 

Last  five 

Baxter 

Ibid.,  33,  127 

57-446 

6 

UBr4 

Three 

Merigold 

Ibid.,  37,  393 

57-447 

7 

CsBr 

Three 

Archibald 

Ibid.,  38,  466 

57-444 

8 

FeBr2 

Two 

Baxter 

Ibid.,  39,  252 

57-443 

9 

CdBra 

Eight 

Hines 

Jour.  Amer.  Chem.  Soc,  28,  783 

57-444 

10 

MnBr2 

Thirteen 

Hines 

Ibid.,  28,  1578 

57-444 

Average,  weighted  according  to  the  number  of  determinations 

57-4443 

Direct  Det 

erminations. 

II 

HBr 

Two 

Richards 

Proc.  Amer.  Acad.,  28,  17,  18 

57-445 

12 

NH4Br 

One 

Richards 

Ibid.,  30,  380 

57-446 

13 

HBr 

Two 

Richards 

Ibid.,  31,  165 

57-444 

14 

NHiBr 

One 

Cushman 

Ibid.,  33,  106 

57-445 

I."; 

NH4Br 

One 

Baxter 

Ibid.,  33,  122 

57-444 

16 

NH^Br 

Two 

Baxter 

Ibid.,  34,  353 

57-447 

17 

NHiBr 

Three 

Baxter 

Ibid.,  39,  250 

57-444 

18 

NHiBr 

One 

Hines 

Jour.  Amer.  Chem.  Soc,  28,  1565 

57-443 

Average,  weighted  accor 

ding  to  the  nun 

iber  of  determinations 

57-4447 

Dumas  ^  by  heating  silver  bromide  in  chlorine  found  the  values  80.02,  79.87, 
and  79.94. 

In  computing  the  atomic  weight  of  bromine  from  these  data,  great  weight  is 
always  given  to  Stas's  determinations,  the  value  79,918  being  usually  assumed 
as  the  most  probable  one  for  the  constant  in  question.  Certainly,  as  pointed 
out  by  Richards,^  the  true  value  must  lie  between  79.91  and  79.92.  Clarke 
calculates  the  value  79.912  as  the  weighted  average  of  the  different  investiga- 
tions previous  to  Scott's.^ 


^  (Euvres  Completes,  1,  81.  *  Jour.  Chem.  Soc.  Trans.,  79, 147  (1901). 

»  Ann.  Chem.  Pharm.,  113,  20  (i860).         *  Proc.  Amer.  Phil.  Soc,  43, 119  (1904). 
^  A  Recalculation  of  Atomic  Weights,  Smith.  Misc.  Coll.,  1897. 


A  REVISION   OF  THE   ATOMIC  WEIGHT   OF  BROMINE.  53 

Considerable  uncertainty  exists  as  to  the  purity  of  the  materials  employed 
in  much  of  the  foregoing  work.  Richards  and  Wells  ^  have  already  exhaustively 
investigated  the  various  methods  of  preparing  pure  silver,  and  have  found  that 
while  it  is  a  comparatively  simple  matter  to  free  this  substance  from  metallic 
impurities,  the  absence  of  gaseous  impurities  is  by  no  means  so  easy  to  secure. 
Oxygen  may  be  eliminated  best  by  fusion  in  an  atmosphere  of  pure  hydrogen 
gas,2  or  by  prolonged  fusion  in  a  vacuum,  while  a  lime  boat  was  found  to  be  the 
most  suitable  support  for  the  silver  during  fusion. 

In  most  of  the  experiments  cited  on  page  52,  one  of  the  final  steps  in  the 
purification  of  the  silver  was  fusion  of  electrolytic  crystals  on  lime,  in  many 
cases  in  a  vacuum,  but  without  especial  care  to  prolong  the  fusion.  Silver  pre- 
pared in  this  way  was  found  by  Richards  and  Wells  to  contain  traces  of  oxy- 
gen, derived  from  silver  nitrate  occluded  by  the  electrolytic  crystals.  In  cases 
8,  9,  10,  17,  and  18,  however,  the  silver  was  fused  in  hydrogen.  Richards  and 
Wells  showed  also  that  Stas's  silver  contained  at  least  o.oi  per  cent  of  impurity, 
since  it  yielded  o.oi  per  cent  less  silver  chloride  than  their  purest  silver.^  Scott's 
silver  in  three  cases  was  merely  heated,  not  fused,  in  hydrogen,  and  in  two  of 
the  others  was  fused  before  a  blowpipe  on  calcic  phosphate.  In  one  experiment 
only  the  metal  was  fused  on  lime.  No  details  are  given  as  to  the  purification 
of  the  silver  used  by  Marignac. 

Bromine  also  may  be  freed  from  impurities  only  with  some  difficulty.  Ex- 
perience in  this  laboratory  has  shown  that  chlorine  may  be  eliminated  most 
conveniently  by  distilling  or  precipitating  the  bromine  from  solution  in  a  bro- 
mide. One  such  distillation  is  sufficient  to  remove  chlorine  completely  only 
when  the  substance  is  initially  comparatively  pure.  If,  however,  the  process 
is  repeated  by  converting  a  portion  of  the  partially  purified  product  into  a  bro- 
mide, and  dissolving  the  remainder  of  the  bromine  in  this  comparatively  pure 
bromide,  the  chlorine  is  eliminated  so  completely  that  further  repetition  of 
this  process  has  no  apparent  effect.^  The  removal  of  iodine  may  be  easily 
effected  by  converting  the  bromine  into  hydrobromic  acid  or  a  soluble  bromide, 
and  boiling  the  solution  with  a  small  quantity  of  free  bromine.  Here  again  it  is 
well  to  repeat  the  process  several  times,  since  the  reaction  betwteen  free  bro- 
mine and  the  iodine  ion,  like  that  between  free  chlorine  and  the  bromine  ion, 
is  undoubtedly  incomplete. 

The  greater  part  of  the  results  given  on  page  52  were  obtained  with  bro- 
mine which  had  been  purified  with  due  observance  of  these  precautions.  Of 
the  other  investigators,  Stas  seems  to  have  been  the  only  one  to  use  sufficient 

*  Pub.  Car.  Inst.  No.  28,  16;  Jour.  Amer.  Chem.  Soc,  27,  472;  Zeit.  anorg.  Ghent.,  47,  70. 
2  Baxter:  Proc.  Amer.  Acad.,  39,  249  (1903);  Zeit.  anorg.  Chem.,  38,  232  (1904). 

^  Loc.  cit.,  page  62. 

*  Attention  has  already  been  called  to  these  points  by  Richards  and  Wells:  Proc.  Amer. 
Acad.,  41,  440  (1906);  Zeit.  physikal.  Chem.,  56,  354. 


54  RESEARCHES   UPON  ATOMIC  WEIGHTS. 

pains  to  secure  purity  of  the  bromine.  Stas  removed  iodine  by  shaking  potas- 
sium bromide  several  times  with  free  bromine  and  carbon  disulphide,  and  in 
the  course  of  the  prolonged  purification  distilled  the  bromine  twice  from  solu- 
tion in  a  bromide.  Marignac's  purification  consisted  solely  in  crystallization 
of  barium  bromate  and  Scott's  in  distillation  of  hydrobromic  acid. 

Of  the  methods  employed  in  these  early  determinations,  that  involving  the 
analysis  of  metallic  halides  is  least  suited  for  the  purpose,  on  accotmt  of  the 
danger  of  occlusion  of  metallic  salts  by  the  precipitated  silver  bromide.  That 
such  an  error  actually  exists  to  a  slight  extent  is  shown  by  the  fact  that  the 
average  of  the  "indirect"  determinations  is  slightly  larger  than  the  average  of 
the  "direct"  determinations.  Obviously,  if  silver  bromide  is  precipitated  by 
means  of  either  ammonium  bromide  or  hydrobromic  acid,  occluded  ammonium 
salts  or  free  acids  can  be  easily  expelled  by  fusion  of  the  bromide.  This  pre- 
caution was  observed  in  most  of  the  determinations  recorded  on  page  52,  and  is 
absolutely  essential  for  the  complete  elimination  of  water  from  the  salt.  Stas 
and  Marignac  both  fused  the  silver  bromide  in  their  syntheses,  but  this  opera- 
tion was  omitted  by  Scott,  who  dried  the  bromide  at  180°.  Scott's  statement 
that  the  loss  on  fusion  of  silver  bromide  which  had  been  dried  at  180°  was  due 
to  the  presence  of  asbestos  is  contradicted  by  the  experiments  recorded  later  in 
this  paper,  in  which  the  loss  on  fusion  amounted  to  about  o.oi  per  cent  in  the 
case  of  silver  bromide  which  had  been  dried  in  a  similar  fashion  and  which  was 
almost  entirely  free  from  asbestos. 

From  this  brief  discussion  of  the  more  important  errors  which  may  have  in- 
fluenced previous  determinations  of  the  atomic  weight  of  bromine,  it  is  evident 
that  some  uncertainty  still  exists  as  to  the  true  value  of  this  constant.  In  the 
hope  of  throwing  new  light  upon  the  subject,  experiments  were  carried  out  by 
two  of  the  methods  outlined  above,  with  especial  precautions  to  insure  purity 
of  materials  and  to  eliminate  known  possible  errors  in  the  experimental  methods. 

Both  the  methods  chosen  —  S)nithesis  of  silver  bromide  from  a  weighed 
amount  of  silver,  and  conversion  of  silver  bromide  into  silver  chloride  —  have 
already  been  recently  tested  in  this  laboratory ,1  and  have  been  found  to  be  at 
least  as  satisfactory  as  any. 

PURIFICATION   OF   MATERIALS. 
BROMINE. 

In  purifying  bromine  for  this  research,  the  principles  set  forth  on  page  53 
of  this  paper  were  appHed;  but  in  some  cases  the  purifying  processes  were  re- 
peated after  the  product  was  apparently  pure,  in  order  to  make  certain  that 
further  treatment  had  no  effect. 

^  Baxter:  Proc.  Amer.  Acad.,  40,  419;  41,  73;  Jour.  Amer.  Chem.  Soc,  26, 1577;  27,  876; 
Zeit.  anorg.  Chem.,  ^Z,  T-A]  46,36;  Richards  and  Wells:  Pj<&.  Car. /«rf.,  No.  28; /owr.^wer. 
Chem.  Soc,  27,  459;  Zeit.  anorg.  Chem.,  47,  56. 


A  REVISION   OF   THE   ATOMIC  WEIGHT   OF   BROMINE.  55 

Sample  I  was  first  completely  dissolved  in  calcic  bromide  which  had  been 
made  from  about  one-third  of  the  original  material  by  means  of  lime  and  am- 
monia, and  was  then  distilled  from  the  solution.  The  product  was  covered 
with  several  times  its  volume  of  water,  and  was  converted  into  hydrobromic 
acid  by  means  of  pure  hydrogen  sulphide  which  had  been  generated  from  fer- 
rous sulphide  with  dilute  sulphuric  acid,  and  which  had  been  thoroughly  washed 
with  water.  After  filtration  from  the  precipitated  sulphur  and  bromide  of  sul- 
phur, the  acid  was  boiled  for  some  time,  with  occasional  addition  of  small  quan- 
tities of  recrystallized  potassium  permanganate  to  eliminate  the  iodine.  Finally 
the  residual  hydrobromic  acid  was  heated  with  an  equivalent  amount  of  re- 
crystalHzed  permanganate,  and  the  bromine  was  condensed  in  a  flask  cooled 
with  ice. 

Sample  II  was  first  converted  into  hydrobromic  acid  by  means  of  red  phospho- 
rus and  water,  and  the  hydrobromic  acid  was  then  distilled,  after  having  been 
boiled  with  an  excess  of  bromine.  An  equivalent  amoimt  of  permanganate  was 
added,  and  the  bromine  liberated  was  separated  from  the  solution  by  distilla- 
tion. About  one-fourth  of  the  product  was  next  transformed  into  calcic  bro- 
mide by  means  of  ammonia  and  lime  which  was  free  from  chloride,  and  the  re- 
maining three-fourths  of  the  bromide  were  dissolved  in  the  calcic  bromide  and 
distilled.  Still  a  third  distillation  from  a  bromide  was  carried  out  by  reducing 
the  product  of  the  second  distillation  with  hydrogen  sulphide  and  subsequently 
oxidizing  the  hydrobromic  acid  with  the  purest  recrystallized  potassium  per- 
manganate, after  boiling  the  acid  with  several  small  portions  of  permanganate 
to  eliminate  last  traces  of  iodine. 

Sample  III  was  obtained  by  preparing  calcic  bromide  from  a  portion  of 
Sample  II  and  distilling  the  remainder  of  Sample  II  from  solution  in  this 
bromide. 

In  the  case  of  Sample  IV  the  processes  of  reduction  to  hydrobromic 
acid  with  hydrogen  sulphide  and  oxidation  of  the  hydrobromic  acid  with  pure 
permanganate  were  four  times  repeated.  After  each  reduction  the  hydro- 
bromic acid  was  boiled  with  free  bromine  to  remove  iodine. 

Sample  V  was  three  times  reduced  with  hydrogen  sulphide  and  oxidized  with 
permanganate.  One-fourth  the  product  was  converted  into  calcic  bromide  and 
the  remainder  was  dissolved  in  this  calcic  bromide  and  distilled. 

Thus  Sample  I  was  twice  distilled  from  a  bromide;  Sample  II  was  treated 
three  times  in  the  same  way;  and  Samples  III,  IV,  and  V  four  times. 

Shortly  before  use  each  sample  was  distilled  and  converted  into  ammonium 
bromide  by  slow  addition  to  an  excess  of  redistilled  ammonium  hydroxide. 
The  solution  was  then  boiled  to  expel  the  excess  of  ammonia. 

SILVER. 

Several  different  samples  of  silver  were  employed,  many  of  which  have 
already  been  used  in  atomic  weight  researches  in  this  laboratory,  and  have 


S6  RESEARCHES   UPON  ATOMIC  WEIGHTS. 

shown  evidence  of  great  purity.  For  details  concerning  the  purification  the 
papers  referred  to  should  be  consulted. 

Sample  A  was  employed  in  a  determination  of  the  atomic  weight  of 
iodine.'  This  specimen  had  been  twice  precipitated  as  chloride  and  once 
electrolyzed. 

Sample  B  was  used  in  experiments  upon  the  atomic  weight  of  iodine  ^  and  of 
manganese.'  It  was  precipitated  once  as  chloride,  electrolyzed  once,  and  finally 
precipitated  as  metal  with  ammonium  formate. 

Sample  C  also  was  employed  in  a  determination  of  the  atomic  weight  of  man- 
ganese, and  was  purified  by  recrystallizing  silver  nitrate,  7  times  from  nitric 
acid  and  5  times  from  aqueous  solution.  Finally  the  silver  nitrate  was  reduced 
by  means  of  ammonium  formate. 

Sample  D  was  prepared  for  the  determination  of  the  atomic  weights  of  cad- 
mium ^  and  manganese,  by  precipitation  as  chloride,  precipitation  with  am- 
monium formate,  and  electrolysis. 

Sample  E  was  first  purified  in  part  by  precipitation  as  chloride,  in  part  by 
precipitation  with  ammonium  formate.  The  combined  material  was  then 
subjected  to  two  electrolyses. 

In  all  cases  the  electrolytic  crystals  were  fused  in  a  boat  of  the  purest  lime, 
contained  in  a  porcelain  tube,  in  a  current  of  electrolytic  hydrogen.  After  the 
buttons  had  been  cleansed  with  dilute  nitric  acid  and  dried  at  200°,  they  were 
cut  into  fragments  of  from  4  to  8  gm.  either  by  means  of  a  clean  chisel  and  anvil 
or  with  a  fine  jeweller's  saw.  The  latter  method  was  employed  in  the  case  of 
samples  D  and  E,  because  it  proved  easier  completely  to  free  the  silver  from 
surface  contamination  with  iron  by  etching  the  fragments  with  nitric  acid,  than 
when  a  chisel  was  used.  The  cleansing  process  with  nitric  acid  was  repeated 
until  the  solution  thus  obtained,  after  precipitation  with  hydrochloric  acid  and 
evaporation,  proved  free  from  iron.  That  every  trace  of  iron  could  be  removed 
by  this  treatment  was  proved  by  testing  for  iron  the  evaporated  filtrates  from 
several  of  the  analyses  subsequently  recorded  in  this  paper.  Negative  results 
were  obtained  in  all  cases. 

After  thorough  washing  with  water  and  drying  at  100°,  the  pieces  of  metal 
were  heated  to  about  400°  in  a  vacuum,  and  were  preserved  over  solid  potas- 
sium hydroxide  in  a  desiccator. 

1  Baxter:  Proc.  Amer.  Acad.,  40,  420  (1904);  Jour.  Amer.  Chem.  Soc,  26, 1578;  Zeit.  anorg. 
Chem.,  43,  15.      (See  page  92.) 

2  Baxter:  Ibid.,  41,  79  (1905);  Joiir.  Amer.  Chem.  Soc,  27,  881;  Zeit.  anorg.  Chem.,  46, 
42.     (See  page  108.) 

'  Baxter  and  Hines:  Jour.  Amer.  Chem.  Soc,  28, 1560  (1906);  Zeit.  anorg.  Chem.,  51,  207. 
(See  page  37.) 

*  Baxter  and  Hines:  Jour.  Amer.  Chem.  Soc,  28,  772  (1906);  Zeit.  anorg.  Chem.  4g,  417. 
(See  page  6.) 


A  REVISION   OF    THE   ATOMIC   WEIGHT    OF   BROMINE.  57 

SYNTHESIS    OF    SILVER    BROMIDE. 

The  ratio  of  silver  to  silver  bromide  was  determined  as  follows:  Weighed 
quantities  of  silver  were  dissolved  in  the  purest  redistilled  nitric  acid  diluted 
with  an  equal  volume  of  water,  in  the  dissolving  flask  described  on  page  12, 
Next,  the  acid  solution  of  the  silver  was  diluted  with  an  equal  volume  of  water, 
and  was  heated  until  free  from  nitrous  acid  and  oxides  of  nitrogen.  After  still 
further  dilution,  the  solution  was  added  slowly  with  constant  agitation  to  a 
dilute  solution  of  an  excess  of  ammonium  bromide  in  a  glass-stoppered  precipi- 
tating flask,  and  the  whole  was  violently  shaken  for  some  time  to  promote  coag- 
ulation. By  adding  the  silver  solution  to  the  bromide,  occlusion  of  silver  nitrate 
was  almost  wholly  precluded.  In  some  experiments  the  solutions  were  as  dilute 
as  twentieth  normal,  in  others  as  concentrated  as  fourth  normal.  The  final  re-' 
suits  seem  to  be  independent  of  the  concentration  of  the  solutions.  At  the 
end  of  about  24  hours  the  flask  with  its  contents  was  again  shaken,  and  then  it 
was  allowed  to  stand  until  the  supernatant  liquid  was  perfectly  clear.  The  pre- 
cipitate of  silver  bromide  was  collected  upon  a  weighed  Gooch  crucible,  after 
thorough  washing  by  decantation  with  water,  and  was  dried  in  an  electric  oven, 
first  for  several  hours  at  130°,  finally  for  about  14  hours  at  180°.  Then  it  was 
cooled  and  weighed. 

The  operations  of  precipitation  and  filtration  were  performed  in  a  large  cup- 
board lighted  with  red  light,  and  if  the  flask  was  taken  out  of  this  cupboard  it 
was  enveloped  in  several  thicknesses  of  black  cloth. 

Even  after  the  prolonged  drying,  traces  of  moisture  were  retained  by  the 
salt,  and  could  be  expelled  only  by  fusion.  This  was  done  by  transferring  the 
bulk  of  the  silver  bromide,  freed  as  completely  as  possible  from  asbestos,  to  a 
small  porcelain  crucible  which  was  weighed  with  its  cover.  The  silver  bromide 
was  then  fused  by  heating  the  small  crucible,  contained  in  a  large  crucible  to 
prevent  direct  contact  with  the  flame  of  the  burner.  A  temperature  much  above 
the  fusing  point  of  silver  bromide  was  avoided  so  that  volatilization  of  the  salt 
could  not  take  place.  This  treatment  must  have  eliminated  occluded  ammon- 
ium salts  as  well  as  water.  Finally,  in  order  to  convert  any  occluded  silver  ni- 
trate, metallic  silver,  or  silver  sub-bromide  into  silver  bromide,  the  salt  was  again 
fused  in  a  current  of  dry  air  containing  bromine  vapor.  This  treatment  seldom 
produced  any  measurable  effect  either  upon  the  weight  or  the  appearance  of  the 
salt,  which  was  perfectly  transparent  and  of  a  light  yellow  color,  even  after  the 
first  fusion  in  air. 

A  few  shreds  of  asbestos  displaced  from  the  crucible,  together  with  an  occa- 
sional trace  of  silver  bromide  which  escaped  the  crucible,  were  collected  upon  a 
tiny  filter  paper,  which  was  then  ignited  in  a  porcelain  crucible.  Before  weighing, 
the  ash  was  either  treated  with  a  drop  of  nitric  and  hydrobromic  acids  and  again 
heated,  or  else  was  heated  for  some  minutes  in  a  current  of  air  and  bromine. 

The  filtrate  and  washings  were  evaporated  to  small  bulk.  The  precipitating 
flask  and  all  other  glass  vessels  used  in  the  analysis  were  rinsed  with  ammonia 


58 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


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A  REVISION   OF  THE  ATOMIC  WEIGHT   OF   BROMINE.  59 

and  the  rinsings  were  added  to  the  evaporated  filtrate  and  wash-waters.  The 
whole  was  then  tested  in  a  nephelometer  for  silver  and  the  quantity  found  was 
estimated  by  comparison  with  standard  silver  solutions.  In  most  cases  the 
correction  thus  obtained  was  less  than  o.i  mg. 

The  asbestos  which  formed  the  felt  in  the  Gooch  crucible,  after  having  been 
shredded,  was  digested  for  some  hours  with  aqua  regia  and  was  then  thor- 
oughly washed  with  water.  Before  the  empty  crucible  was  weighed,  the  felt  was 
ignited  with  a  Bunsen  burner.  Crucibles  thus  treated  and  then,  after  being 
moistened  with  water,  again  heated  to  iSo**,  did  not  change  in  weight. 

In  the  table  on  page  58  are  cited  all  the  analyses  which  were  completed  with- 
out accident.  Vacuum  corrections  of  —0.000031  for  every  apparent  gram  of 
silver  and  of  4-0.000041  for  every  apparent  gram  of  silver  bromide  are  applied.^ 

The  platinum-plated  brass  weights  were  standardized  from  time  to  time  and 
were  found  to  retain  their  original  values  within  a  very  few  hundredths  of  a 
milligram  in  all  cases. 

CONVERSION    OF   SILVER   BROMIDE   INTO   SILVER   CHLORIDE. 

The  ratio  of  silver  bromide  to  silver  chloride  was  determined  much  as  de- 
scribed in  previous  papers  upon  the  atomic  weight  of  iodine.^  Pure  silver 
bromide  was  prepared  by  precipitation  of  silver  nitrate  with  an  excess  of 
ammonium  bromide.  The  silver  employed  was  purified  either  by  precipitation 
as  chloride  and  reduction  with  invert  sugar,  or  by  electrolysis,  or  by  precipita- 
tion with  ammonium  formate.  The  metal  was  then  fused  before  a  blowpipe 
upon  a  crucible  of  the  purest  lime,  and  the  buttons  were  thoroughly  cleansed 
with  nitric  acid.  No  further  purification  was  considered  necessary  since  the 
weight  of  the  metal  was  of  no  consequence. 

After  the  silver  bromide  had  been  washed  by  decantation  with  water,  in  some 
cases  it  was  collected  in  a  Gooch  crucible  in  which  a  disk  of  filter  paper  was 
employed  instead  of  asbestos,  and  after  drying  at  100°  it  was  carefully  sepa- 
rated from  the  filter  paper.  In  other  cases  the  precipitate  was  transferred  to  a 
platinum  dish,  and  was  drained  with  a  platinum  reverse  filter  ^  with  a  disk  of 
filter  paper.  In  still  others  a  platinum  Gooch  crucible  with  small  holes  was 
found  to  belsufl&ciently  effective  as  a  filtering  medium  without  the  use  of  either 
asbestos  or  filter  paper. 

Before  being  weighed  the  silver  bromide  was  fused  in  a  current  of  air  saturated 
with  bromine  in  a  weighed  quartz  crucible.  The  air  was  purified  by  passing 
successively  over  beads  moistened  with  silver  nitrate  solution,  over  sodium 
carbonate,  and  finally  over  concentrated  sulphuric  acid  which  had  been  heated 
to  its  boiling-point  with  a  small  quantity  of  recrystallized  potassium  dichromate 
to  eliminate  volatile  and  oxidizable  impurities,    The  air  was  then  passed 

*  See  page  41. 

^  Baxter:  Proc.  Amer.  Acad.,  40,  432  (1904);  41,  75  (1905);  Jour.  Amer.  diem.  Soc,  26, 
1590;  37,  876;  Zeit.  anorg.  Chem.,43,  27;  46,  36.     (See  page  102.) 
'  Cooke:  Proc.  Amer.  Acad.,  12,  121  (1876). 


6o  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

through  dry  bromine  in  a  small  bulb.  This  apparatus  was  constructed  entirely 
of  glass  with  ground  joints.  The  tube  which  conducted  the  gases  into  the  cru- 
cible passed  through  a  Rose  crucible-cover  of  glazed  porcelain  in  all  experiments 
except  analyses  28  to  31,  in  which  a  quartz  cover  was  employed.  The  quartz 
crucibles  were  always  contained  in  large  porcelain  crucibles  while  being  heated. 
They  remained  almost  absolutely  constant  in  weight  during  the  experiments. 
The  bromine  was  in  each  case  a  portion  of  the  sample  from  which  the  silver 
bromide  had  been  made. 

Next  the  bromide  was  heated  barely  to  fusion  in  a  slow  current  of  chlorine, 
generated  by  the  action  of  hydrochloric  acid  upon  manganese  dioxide,  and 
dried  by  means  of  concentrated  sulphuric  acid.  The  apparatus  for  this  pur- 
pose also  was  constructed  wholly  of  glass.  When  the  bromine  was  apparently 
completely  displaced,  the  silver  chloride  was  heated  in  the  air  for  a  few  min- 
utes to  expel  dissolved  chlorine,  and  then  was  cooled  and  weighed.  A  repetition 
of  the  heating  in  chlorine  seldom  affected  the  weight  of  the  salt  more  than  a 
few  hundredths  of  a  milligram,  although  occasionally  a  third  heating  was 
necessary  to  effect  this  result. 

That  no  loss  of  silver  chloride  by  volatiHzation  took  place  is  certain  for  two 
reasons.  In  the  first  place  the  cover  of  the  crucible  and  the  delivery- tube  for 
the  bromine,  when  rinsed  with  ammonia  and  the  solution  treated  with  a  sUght 
excess  of  hydrochloric  acid,  gave  no  visible  opalescence  in  the  nephelometer. 
In  the  second  place  the  weight  of  the  chloride  became  constant  without  diffi- 
culty. It  has  already  been  shown  that  silver  chloride  which  has  been  fused 
in  chlorine,  if  subsequently  heated  in  air,  retains  no  excess  of  chlorine.^ 

The  following  vacuum  corrections  were  applied:  silver  bromide,  -f-  0.000041 ; 
silver  chloride  +  0.000071.^     (See  table  on  page  61.) 

RESULTS   AND   DISCUSSION. 

Aside  from  the  close  agreement  of  all  the  results  of  Series  I,  the  fact  is  to  be 
emphasized  that  of  the  last  seven  analyses,  which  were  consecutive,  only  two 
differ  from  the  average  of  the  series,  79.916,  by  as  much  as  o.ooi  unit.  Fur- 
thermore, there  is  no  evidence  of  any  dissimilarity  in  the  different  preparations 
of  bromine.  Material  which  has  received  only  two  distillations  from  a  bromide 
gives  values  no  lower  than  bromine  which  has  been  thus  treated  four  times. 
The  various  specimens  of  silver  also  show  no  difference  in  purity. 

In  the  case  of  Series  II,  the  extreme  variation  of  the  results  is  only  0.004  unit, 
and  only  one  of  the  13  experiments  yielded  a  value  which  differs  from  the  aver- 
age by  more  than  o.ooi  unit. 

Finally,  the  difference  between  the  averages  of  Series  I  and  II  is  only  0.0007 
unit.    It  is  extremely  unlikely  that  constant  errors  could  have  affected  both 

^  Baxter:  Proc.^wer.  ^ca^/., 40,  432  (1904);  Jour.  Amer.  Chem.  Soc. ,26,  isg^;  Zeit.anorg. 
Chem.,  43,  29.     (See  page  103.) 
^  See  page  41. 


A  REVISION   OF   THE   ATOMIC  WEIGHT   OF   BROMINE. 


6i 


series  equally,  so  that  this  striking  agreement  is  strong  proof  that  both  series 
are  free  from  such  errors. 

The  Atomic  Weight  of  Bromine.    Series  II.    AgBr:AgCl. 
Ag  =  107.880  CI  =  35-457- 


No.  of 
analysis. 

Sample 

of 
bromine. 

Weight  of 

silver  bromide 

in  vacuum. 

Weight  of 

silver  chloride 

in  vacuum. 

^■"»!ff 

Atoniic 
weight  of 
bromine. 

19 
20 
21 

22 
23 

24 

25 
26 
27 

28 
29 

30 
31 

II 
II 
II 

IV 
IV 

V 
V 
V 
V 

I 
I 

III 
III 

Total 

gm. 

8.03979 

8.57738 

13.15698 

12.71403 
13.96784 

13.0S168 
12.52604 
II.I1984 

8.82272 

II.93192 
12.53547 

17.15021 
10.31852 

153.94242 

gm. 

6.13642 

6.54677 

10.04221 
Average 
9.70413 

10.66116 
i\verage 
9.98469 
9-56059 
8.48733 
6.73402 

Average 
9.10721 
9.56767 

Average 
13.09009 
7.87572 

Average 

131.0176 
131.0170 
131.0168 

79.916 
79-915 
79-915 

131.0171 

79.915 

131.0167 
131.0162 

79.915 
79.914 

131.0164 

79-915 

131.0174 
131.017s 
131.0170 
131.0172 

79.916 
79.916 

79-915 
79.916 

131.0173 

79.916 

131.0162 
131.0190 
131.0176 

79.914 
79.918 

79.916 

131.0167 
131.0168 

79-915 
79-915 

131.0168 

79-915 

117.49801 

131.0170 

79-915 

Average  of  all  13 
Averacre  of  .Spripfs 

sxperiments 

131.0171 

79-915 
79.916 

I  and  II 

It  has  already  been  pointed  out  that  the  average  of  Stas's  syntheses,  79.917, 
probably  represents  with  considerable  accuracy  the  atomic  weight  of  bromine, 
and  that  certainly  his  determinations  are  more  accurate  than  those  of  later 
experimenters.  His  syntheses  are  few  in  number,  however,  and  differ  among 
themselves  by  several  thousandths  of  a  unit,  so  that  they  do  not  define  within 
this  amount  the  constant  in  question.  Their  average,  however,  confirms  the 
value  obtained  in  this  paper.  From  all  the  experiments  here  described  the 
number  79.916  seems  to  be  the  most  probable  value  for  the  atomic  weight  of 
bromine,  if  silver  has  the  atomic  weight  107.880.  If  silver  is  taken  at  107.870, 
bromine  becomes  79.909. 

In  conclusion,  attention  may  be  called  to  the  fact  that  a  diminution  in  the 
atomic  weight  of  bromine  referred  to  silver  raises  slightly  all  atomic  weights 
resulting  from  the  analysis  of  metallic  bromides  by  precipitation  with  silver, 

I  am  deeply  indebted  to  the  Cyrus  M.  Warren  Fund  for  Research  in  Harvard 
University  for  assistance  in  pursuing  this  investigation. 


V. 

A  REVISION   OF   THE   ATOMIC  WEIGHT    OF 

LEAD. 

777^  ANALYSIS   OF  LEAD   CHLORIDE. 


By  Gregory  Paul  Baxter  and  John  Hunt  Wilson. 


Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  43,  365  (1907). 

Journal  of  the  American  Chemical  Society,  30,  187  (1908). 

Zeitschrift  fiir  anorganische  Chemie,  57,  174  (1908). 

Chemical  News,  98,  64,  78  (1908). 

Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF  LEAD. 

THE   ANALYSIS   OF   LEAD   CHLORIDE. 


INTRODUCTION. 


Although  lead  is  one  of  the  most  common  elements,  its  atomic  weight  has  te- 
ceived  comparatively  little  attention,  the  value  at  present  accepted  being  based 
almost  wholly  upon  the  work  of  Stas.^  Of  the  earlier  determinations  of  this 
constant  those  of  Dobereiner  ^  and  Longchamps  ^  can  hardly  be  considered  as 
possessing  other  than  historic  interest.  The  first  results  which  can  lay  claim  to 
accuracy  are  those  of  Berzelius,^  who  obtained  values  ranging  from  206.7  to 
207.3  by  reduction  of  litharge  in  a  current  of  hydrogen.  Berzelius  also  S3Tithe- 
sized  the  sulphate  from  metallic  lead  with  the  result  207.0.^  Shortly  after, 
Turner  ®  criticized  the  first  method  employed  by  Berzelius  and  attributed  the 
irregularity  of  his  results  to  the  action  of  lead  oxide  on  the  silicious  matter  of 
the  tube  at  the  temperature  employed  in  the  reduction.  By  the  conversion  of 
both  the  metal  and  the  oxide  into  sulphate  Turner  in  a  painstaking  research 
deduced  the  values  207.0  and  207.6  respectively,  and  by  converting  the  nitrate 
into  sulphate,  204.2.  Marignac  ^  converted  metallic  lead  into  the  chloride  by 
heating  in  a  stream  of  chlorine  and  obtained  the  result  207.42.  Both  Marig- 
nac ^  and  Dumas  ^  analyzed  lead  chloride.  Marignac,  who  dried  the  salt  at 
200°,  by  titration  against  silver  found  the  atomic  weight  of  lead  to  be  206.81, 
and  from  the  ratio  of  lead  chloride  to  silver  chloride,  206.85.  Dumas  subse- 
quently showed  that  lead  chloride,  even  when  dried  at  250°,  retains  moisture 
and  is  somewhat  basic,  and  in  one  analysis,  in  which  corrections  are  applied  for 
these  errors,  found  a  somewhat  higher  value,  207.07,  as  was  to  be  expected. 
Chloride  analyses  by  early  investigators  are,  however,  to  be  universally  dis- 
trusted, owing  to  neglect  of  the  very  considerable  solubility  of  silver  chloride, 
thus  producing  too  low  results. 

Stas's  work  upon  the  syntheses  of  lead  nitrate  and  sulphate  from  the  metal 
is  undoubtedly  the  most  accurate  contribution  upon  the  subject,^"  although  a 

1  Earlier  work  on  the  atomic  weight  of  lead  has  been  carefully  summarized  by  Clarke. 
Smithsonian  Miscellaneous  Collections,  Constants  of  Nature,  "A  Recalculation  of  the  Atomic 
Weights,"  1910.  The  earlier  results  have  been  recalculated  on  the  basis  of  the  following  atomic 
weights:  0  =  16.00;  N  =  14.01;  S  =  32.07;  CI  =  35.46;  Ag  =  107.88. 

*  Schweig.  Jour.,  17,  241  (1816).  '  Ann.  Chim.  Phys.,  34,  105  (1827). 

*  Pogg.  Ann.,  19,  314  (183c).  ^  Lehrbuch,  sth  ed.,  3, 1187  (1845). 
^  Phil.  Trans.,  527  (1833).  '  Lieh.  Ann.,  59,  289  (1846). 

^  Jour.  Prakt.  Chem.,  74,  ziS  (1858).  ^  Lieb.  Ann.,  113,  35  (i860). 

^^  (Euvres  Completes,  i,  383. 

6s 


66  RESEARCHES    UPON   ATOMIC   WEIGHTS. 

careful  consideration  of  his  work  discloses  minor  defects,  many  of  which  he 
recognizes  himself.  The  metallic  lead  used  in  the  syntheses  was  finally  fused 
under  potassium  cyanide.  Whether  or  not  this  treatment  introduced  impuri- 
ties into  the  metal  is  uncertain.  Stas  himself  suspected  the  presence  of  alkali 
metals.  Since  the  nitrate  could  not  be  dried  above  150°  without  decom- 
position, it  undoubtedly  contained  moisture,  and  Stas  calls  attention  to  this 
point.  The  sulphate  was  made  by  treatment  of  lead  nitrate,  resulting  from 
the  nitrate  syntheses,  with  sulphuric  acid.  The  sulphate  was  dried  finally  at 
dull  redness,  and  was  probably  free,  or  nearly  free,  from  moisture,  although  it 
may  have  contained  traces  of  lead  oxide  resulting  from  occluded  nitrate,  as 
well  as  sulphuric  acid.  Most  of  these  probable  errors  tend  to  lower  the  observed 
atomic  weight,  so  that  Stas's  value  from  the  series  of  nitrate  syntheses,  206.81, 
and  that  from  the  sulphate  series,  206.92,  are  to  be  regarded  as  minimum 
values.  The  reader  of  Stas's  own  account  of  his  work  upon  lead  can  not  fail  to 
be  impressed  with  the  fact  that  he  was  somewhat  dissatisfied  with  the  outcome 
of  his  research.  Mention  should  also  be  made  of  the  work  of  Anderson  and 
Svanberg  ^  on  the  conversion  of  lead  nitrate  into  oxide,  although  the  method 
was  primarily  employed  in  an  endeavor  to  fix  the  atomic  weight  of  nitrogen. 
Their  results  yield  the  value  207.37. 

The  discrepancies  between  the  results  of  these  various  experiments  only  serve 
to  emphasize  the  need  of  a  redetermination  of  the  value  in  question,  and  it  was 
with  this  object  in  view  that  the  work  embodied  in  this  paper  was  undertaken. 

The  search  for  a  suitable  method  for  determining  the  atomic  weight  of  lead 
failed  to  reveal  any  more  promising  line  of  attack  than  those  already  employed 
for  the  purpose.  With  an  element  of  so  high  an  atomic  weight  as  lead,  in  any 
method  involving  the  change  of  one  of  its  compounds  into  another,  errors  which 
may  be  insignificant  with  elements  of  small  atomic  weight  are  magnified  in  the 
calculations  to  undesirable  proportions.  Furthermore,  during  the  following 
investigation,  reduction  of  the  chloride  and  oxide  in  hydrogen  was  investigated 
far  enough  to  show  that  complete  reduction  of  either  compound  was  extremely 
difficult,  if  not  impossible,  without  loss  of  material  from  the  containing  vessel  by 
sublimation,  aside  from  the  fact  that  all  available  material  for  containing  ves- 
sels is  acted  upon  by  either  the  fused  salt  or  the  reduced  metal.  The  elimination 
of  moisture  from  lead  nitrate  or  lead  sulphate  without  decomposition  of  the 
salts  seemed  likely  to  prove  a  stimibling-block  in  the  use  of  these  substances. 
Finally,  in  spite  of  the  slight  solubility  of  lead  chloride,  the  determination  of 
the  chlorine  in  this  salt  by  precipitation  with  silver  nitrate  was  chosen  as  pre- 
senting fewest  difficulties.  In  the  first  place,  the  determination  of  a  halogen  can 
be  effected  with  great  accuracy.  In  the  second  place,  the  elimination  of  mois- 
ture from  lead  chloride  is  an  easy  matter,  since  the  salt  may  be  fused  in  a  plat- 
inum vessel  in  a  current  of  hydrochloric-acid  gas  without  attacking  the  platinum 

*  Ann.  Chim.  Phys.  (3),  9,  254  (1843). 


A  REVISION  OF   THE   ATOMIC   WEIGHT   OF   LEAD.  67 

in  the  least  and  without  the  production  of  basic  salts.  In  the  third  place,  silver 
chloride,  which  has  been  precipitated  from  a  dilute  solution  of  lead  chloride  by 
means  of  silver  nitrate,  was  shown  experimentally  not  to  contain  an  amount  of 
occluded  lead  salt  large  enough  to  be  detected. 

PURIFICATION   OF   MATERIALS, 

Water,  hydrochloric  acid,  and  nitric  acid  were  carefully  purified  by  distilla- 
tion as  described  in  the  preceding  papers.  In  the  preparation  of  pure  silver 
also  the  usual  methods  were  employed,  one  precipitation  as  chloride,  one  as 
metal  by  ammonium  formate,  and  one  by  electrolysis  being  followed  by  the 
final  fusion  in  hydrogen  on  a  boat  of  pure  lime. 

LEAD  CHLORIDE. 

Three  samples  of  lead  chloride  from  two  entirely  different  sources  were  em- 
ployed. Sample  A  was  prepared  from  metallic  lead.  Commercial  lead  was  dis- 
solved in  dilute  nitric  acid,  and  the  solution,  after  filtration,  was  precipitated 
with  a  slight  excess  of  sulphuric  acid.  The  lead  sulphate  was  thoroughly  washed, 
suspended  in  water,  and  hydrogen  sulphide  was  passed  in  until  the  sulphate  was 
almost  completely  converted  into  sulphide.  Next  the  sulphide  was  washed  with 
water,  dissolved  in  hot  dilute  nitric  acid,  and  the  solution  was  freed  from  sulphur 
and  unchanged  sulphate  by  filtration.  The  lead  nitrate  thus  obtained  was 
crystallized  twice,  dissolved  in  water,  and  precipitated  in  glass  vessels  with  a 
slight  excess  of  hydrochloric  acid.  The  chloride  was  washed  several  times  with 
cold  water  and  then  crystallized  from  hot  water  eight  times,  the  last  five  crys- 
tallizations being  carried  out  wholly  in  platinum,  with  centrifugal  drainage 
after  each  crystallization.  In  crystallizing  the  lead  chloride  the  whole  sample 
was  not  dissolved  at  one  time,  but  the  same  mother-liquor  was  used  for  dissolv- 
ing several  portions  of  the  original  salt.  Needless  to  say,  the  chloride  was  not 
exposed  to  contact  with  the  products  of  combustion  of  illuminating  gas,  lest  lead 
sulphate  be  formed. 

Sample  B  was  prepared  from  commercial  lead  nitrate.  This  salt  was  dissolved 
and  crystallized  from  dilute  nitric  acid  once  in  glass  and  six  times  in  platinum 
vessels,  with  centrifugal  drainage.  Hydrochloric  acid  was  then  distilled  into  a 
large  quartz  dish,  and  the  solution  of  the  nitrate  was  slowly  added  with  constant 
stirring  with  a  quartz  rod.  The  chloride  was  freed  from  aqua  regia  as  far  as 
possible  by  washing  with  cold  water,  and  was  once  crystallized  from  aqueous 
solution  in  quartz  dishes  to  remove  last  traces  of  aqua  regia.  Finally  the  salt 
was  crystallized  three  times  in  platinum. 

It  could  reasonably  be  expected  that  both  of  these  samples  were  of  a  high 
degree  of  purity;  nevertheless,  upon  heating  the  salt  in  an  atmosphere  of  hy- 
drochloric acid,  the  salt  itself  turned  somewhat  dark,  and  upon  solution  of  the 
fused  salt  in  water  a  slight  dark  residue  remained.  Although  in  a  few  preliminary 


68  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

experiments  attempts  were  made  to  determine  this  residue  oy  miration  anu 
ignition,  it  was  subsequently  found  that  even  a  small  filter  paper  adsorbs  ap- 
preciable amounts  of  lead  compounds  from  a  solution  of  the  chloride,  which  can 
not  be  removed  by  washing  with  water.  From  0.03  mg.  to  0.13  mg.  of  residue 
were  obtained  in  several  blank  experiments,  by  ignition  of  filters  through 
which  0.5  per  cent  solutions  of  lead  chloride  had  been  passed,  with  subsequent 
very  thorough  washing.  In  order  to  avoid  the  uncertainty  of  this  correc- 
tion, further  attempts  were  made  to  obtain  a  sample  of  the  salt  which  would 
give  a  perfectly  clear  solution  in  water  after  fusion,  and  thus  render  filtra- 
tion unnecessary.  With  this  end  in  view  a  considerable  quantity  of  Sample  A 
was  fused  in  a  large  platinum  boat  in  a  current  of  hydrochloric  acid.  The  fused 
salt  was  powdered  in  an  agate  mortar,  dissolved  in  water  in  a  platinvma  vessel, 
and  the  solution  was  freed  from  the  residue  by  filtration  through  a  tiny  filter 
in  a  platinum  funnel  into  a  platinum  dish,  where  it  was  allowed  to  crystallize. 
This  sample  was  then  twice  recrystallized  with  centrifugal  drainage.  Not- 
withstanding the  drastic  treatment  to  which  it  had  been  subjected,  when  a 
portion  of  this  material  was  fused  in  hydrochloric  acid,  the  same  darkening  as 
before  was  observed,  and  the  same  residue  was  obtained.  The  suspicion  that 
the  difiiculty  was  due  to  dissolving  of  the  filter  paper  by  the  solution  of  the 
salt  ^  led  to  a  second  more  successful  attempt  by  crystallization  from  hydro- 
chloric acid  solution  in  platinum  vessels.  In  this  way  it  was  found  possible 
to  prepare  salt  which  showed  no  tendency  to  darken  upon  heating,  and  which, 
after  fusion,  left  absolutely  no  residue  upon  solution  in  water.  Portions  of 
Samples  A  and  B  were  thus  recrystallized  three  times  more.  Since  these  two 
specimens  of  material  gave  identical  results,  for  two  final  experiments,  portions 
from  each  of  these  samples  were  mixed  and  then  subjected  to  three  additional 
crystalHzations.   This  last  sample  was  designated  Sample  C. 

DRYING  OF   LEAD   CHLORIDE   AND   METHOD   OF   ANALYSIS. 

The  method  of  analysis  did  not  differ  materially  from  that  used  in  the  analy- 
sis of  cadmium  and  manganese  chlorides  (pages  7  and  45).  The  lead  chloride, 
contained  in  a  weighed  platinum  boat,  was  first  fused  in  a  current  of  hydro- 
chloric-acid gas,  and  the  boat  was  transferred  to  its  weighing-bottle  and 
weighed.  On  account  of  the  small  solubility  of  lead  chloride  it  was  a  somewhat 
troublesome  matter  to  dissolve  the  fused  material.  This  was  done  in  most  of 
the  analyses  by  prolonged  contact  with  water  nearly  at  the  boiling  point  in  a 
Jena  glass  flask.  In  the  last  two  analyses,  in  order  to  show  that  no  error  was 
introduced  through  the  solubility  of  the  glass,  the  solution  was  prepared  in 
a  large  platinum  retort  and  was  transferred  to  the  precipitating  flask  only 
when  cold. 

^  Mr,  P.  B.  Goode  in  this  laboratory  has  recently  found  a  similar  difficulty  with  the  chlo- 
rides of  the  alkaline  earths. 


A  REVISION   OF   THE  ATOMIC  WEIGHT   OF   LEAD.  69 

When  the  lead  chloride  was  dissolved,  a  dilute  solution  of  a  very  neariy  equiv- 
alent amount  of  pure  silver  was  added,  and,  after  standing,  the  amounts  of 
chloride  and  silver  were  carefully  adjusted  until  exactly  equivalent  by  means 
of  the  nephelometer.  When  the  exact  end-point  had  been  found,  about  0.2 
gm.  of  silver  nitrate  in  excess  was  added  to  precipitate  dissolved  silver  chloride, 
and  the  precipitate  was  collected  and  determined  upon  a  Gooch  crucible.  Dis- 
solved silver  chloride  in  the  filtrate  and  wash-waters  was  estimated  by  com- 
parison with  standard  solutions  in  the  nephelometer.  Asbestos  displaced  from 
the  Gooch  crucible  was  collected,  and  the  moisture  retained  by  the  dried  pre- 
cipitate was  found  by  loss  of  weight  during  fusion. 

In  order  to  find  out  whether  lead  or  silver  nitrates  were  appreciably  adsorbed 
by  the  filter  paper,  a  solution  containing  lead  nitrate,  silver  nitrate,  and  nitric 
acid  of  the  concentration  of  these  filtrates,  was  passed  through  several  small 
filter  papers,  which  were  then  very  carefully  washed.  In  four  cases,  after  in- 
cineration of  the  papers,  there  was  found,  —  o.ooooi,  -{-  0.00002,  +  0.00003, 
-f-  0.00001  gm.  of  residue,  exclusive  of  ash.  This  correction  is  so  small  that  it 
is  neglected  in  the  calculations.  In  all  the  analyses  the  platinum  boat  behaved 
admirably,  the  loss  in  weight  never  amounting  to  more  than  a  few  hundredths 
of  a  milligram. 

The  balance  used  was  a  short-arm  Troemner,  easily  sensitive  to  0.02  mg. 
The  gold-plated  brass  weights  were  carefully  standardized  to  hundredths  of  a 
milligram.  All  the  weighings  were  made  by  substitution  with  tare  vessels  as 
nearly  like  those  to  be  weighed  as  possible. 

Vacuum  corrections.  The  values  of  the  density  of  lead  chloride  as  given  by 
various  observers  range  from  5.78  to  5.805,^  the  mean  of  the  more  accurate  de- 
terminations being  5.80.  This  gives  rise  to  a  vacuum  correction  of  -j-  0.000062 
for  each  apparent  gram  of  lead  chloride,  the  density  of  the  weights  being  as- 
sumed to  be  8.3 .  The  other  vacuum  corrections  applied  were  for  silver  chloride, 
-f  0.000071,  and  for  silver,  —0.000031. 

All  analyses  which  were  carried  to  a  successful  completion  are  recorded  in  the 
tables  on  page  70. 

RESULTS   AND   DISCUSSION. 

The  close  agreement  of  the  averages  of  the  two  series  is  strong  evidence  that 
no  constant  error,  such  as  occlusion,  affects  the  results.  In  all,  19.55663  gm. 
of  silver  produced  25.98401  gm.  of  silver  chloride,  whence  the  ratio  of  silver 
chloride  to  silver  is  132.865,  a  value  in  close  agreement  with  the  result  132.867 
obtained  by  Richards  and  Wells.  Furthermore,  the  different  samples,  A,  B, 
and  C,  all  give  essentially  identical  results. 

It  appears,  then,  that  if  the  atomic  weight  of  silver  is  taken  as  107.880,  the 
atomic  weight  of  lead  is  207.09,  nearly  0.2  unit  higher  than  the  value  now  in 

*  Landolt-Bomstein-Meyerhoffer,  Tabellen. 


70 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


use.  If  the  atomic  weight  of  silver  is  107.87,  lead  becomes  207.08,  a  number 
still  much  higher  than  that  depending  upon  Stas's  syntheses,  as  is  to  be  ex- 
pected. 

The  Atomic  Weight  of  Lead. 


Series  I.    PbClj:  aAg. 
Ag  =  107.880  CI  =  35-457 


No. 

of 

analysis. 


Sample 

of 
PbCl,. 


Weight  of 

PbClz 
in  vacuum. 


gm. 
4.67691 
3-67705 
4.14110 
4-56988 
5.12287 

3-85844 
4.67244 
3-10317 
4.29613 


Weight  of 

Ag  in 
vacuum. 


gm. 
3.63061 

2-85375 
3-21388 
3-54672 
3-97596 
2.99456 
3.62628 
2.40837 
3-33427 


Weight  of 
Ag  added  or 
subtracted. 


gm. 

—  0.00074 
0.00000 

+0.00020 
0.00000 

—  0.00028 
0.00000 
0.00000 
0.00000 

—0.00020 


Corrected 
weight 
of  Ag. 


gm. 
3.62987 

2-85375 
3.21408 
3-54672 
3-97568 
2.99456 
3.62628 
2.40837 
3-33407 


Atomic 

weight  of 

Pb. 


207.083 
207.093 
207.077 
207.089 
207.105 
207.090 
207.093 
207.092 
207.106 


Average 207.092 


Series  II.    PbCl2:  2AgCl. 


No. 

Sample 

of 

fof 

analysis . 

PbClj. 

10 

A 

II 

A 

12 

B 

13 

B 

14 

C 

IS 

C 

Weight  of 
PbCl,  in 
vacuum. 


gm. 
4.67691 
4.14110 
5.12287 

3-85844 
3-10317 
4.29613 


Weight  of 
AgCl  in 
vacuum. 


gm. 
4.82148 
4.26848 
5.28116 
3-97759 
3-19751 
4.42730 


Loss 

on 

fusion. 


gm. 
O.OOIOO 
0.00020 
0.00054 
0.00035 
0.00045 
0.00020 


Weight 

of 
asbestos. 


gm. 
0.00021 
0.00008 
0.00013 
0.00033 
0.00014 
0.00004 


Wt.  AgCl 
from  wash- 
waters. 


gm. 
0.00204 
0.00180 
0.00197 
0.00192 
0.00189 
0.00268 


Corrected 
weight 
of  AgCl. 


gm. 
4-82273 
4.27016 
5.28272 

3-97949 
3.19909 
4.42982 


Atomic 
weight 
of  Pb. 


207.092 
207.096 
207.085 
207.040 
207.165 
207.108 


Average 207.097 

Average,  rejecting  the  least  satisfactory  analyses,  13  and  14 207.095 

Average  of  Series  I  and  II 207.094 


Vl. 

A   REVISION    OF   THE   ATOMIC   WEIGHT   OF 

ARSENIC. 

THE  ANALYSIS   OF  SILVER   ARSENATE. 


By  Gregory  Paul  Baxter  and  Fletcher  Barker  Coffin. 


Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  44,  179  (1909). 

Journal  of  the  American  Chemical  Society,  31,  297  (1909). 

Zeitschrift  fiir  anorganische  Chemie,  62,  50  (1909). 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF  ARSENIC 

THE  ANALYSIS  OF  SILVER   ARSENATE. 


INTRODUCTION. 


Below  is  a  summary  of  the  previous  work  upon  the  atomic  weight  of  arsenic/ 
the  results  obtained  by  the  several  investigators  having  been  recalculated  with 
the  use  of  the  most  recent  ^  atomic  weights; 


Date. 

Investigator. 

Reference. 

Ratio  determined. 

Result. 

i8i6 

Thomson 

Schweigger's  Jour.,  17, 

421 

2 As:  AsjOb 

76.3s 

1818 

Berzelius 

Pogg.  Ann.,  8,  i 

2AS2O3:  3S02 

75-03 

1845 

Pelouze 

Compt.  Rend.,  20,  1047 

AsClaisAg 

74.93 

I8SS 

Kessler 

Pogg.  Ann.,  95,  204 

3AS2O3:  2K2Cr207 

3AS2O3:  2KCIO8 

74-95 
75-23 

1859 

Dumas 

Ann.  Chim.  Phys.  (3), 

55,  174 

AsCl3:3Ag 

74-87 

185Q 

Wallace 

Phil.  Mag.  (4),  18,  279 

AsBr3:3Ag 

74.20 

1861 

Kessler 

Pogg.  Ann.,  113,  140 

3AS2O3:  2K2Cr207 

75 -oi 

1896 

Hibbs 

Doctoral  Thesis,  Univ. 

of  Penn. 

Na4As207:4NaCl 

74-88 

1902 

Ebaugh 

Jour.  Amer.  Chem.  Soc 

.,  24,  489 

Ag3As04:3AgCl 
Ag3As04:3Ag 
Pb3(As04)2:3PbCl2 
Pb3(As04)2:3PbBr2 

75.02 

74.92 
75-06 
74-88 

A  glance  at  this  rather  discordant  series  of  results  shows  the  necessity  for  a 
redetermination  of  the  atomic  weight  of  arsenic.  Even  in  the  more  recent  in- 
vestigations of  Hibbs  and  Ebaugh  there  exists  an  extreme  variation  of  nearly 
0.2  imit  in  the  averages  of  the  five  series. 

In  this  research  silver  arsenate  was  chosen  for  analysis,  first,  because  the 
compound  is  unchanged  by  moderate  heating,  and  hence  may  be  dried  at  a 
temperature  high  enough  to  expel  all  but  a  very  small  amount  of  moisture. 
In  the  second  place,  silver  compounds  may  be  analyzed  with  great  ease  as  well 
as  accuracy  by  precipitation  of  the  silver  as  silver  halogen  compounds.  Fur- 
thermore, preliminary  experiments  confirmed  the  statement  by  Ebaugh  that 

*  Clarke:  A  recalculation  of  the  atomic  weights,  Smith.  Misc.  Coll.,  Constants  of  Nature, 
Part  V  (1910).  For  an  excellent  critical  discussion  by  Brauner  of  previous  work,  see 
Abegg's  Eandhuch  der  anorganischen  Chemie,  3,  (2),  491  (1907). 

*  O  =  16.00;  Na  =  22.98;  S  =  32.07;  CI  =  35.46;  K  =  39.10;  Cr  =  52.01;  Br  =■ 
79.92;   Ag  =  107.88;  Pb  =  207.09. 

73 


74  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

it  is  possible  completely  to  convert  the  arsenate  into  chloride  by  heating  in  a 
current  of  hydrochloric-acid  gas.  Such  a  process  has  the  advantage  that  no 
transfer  of  material  is  involved. 

THE    PREPARATION    OF    TRISILVER   ARSENATE. 

All  the  samples  of  silver  arsenate  were  prepared  by  adding  to  a  j&f  teenth  nor- 
mal solution  of  silver  nitrate  a  solution  of  similar  concentration  of  an  equiva- 
lent amount  of  an  arsenate  of  sodium  or  ammonium,  the  differences  between  the 
different  samples  consisting  chiefly  in  the  nature  of  the  soluble  arsenate  em- 
ployed. Precipitation  was  carried  out  in  a  room  lighted  only  with  ruby  light. 
After  the  silver  arsenate  had  been  washed  by  decantation  many  times  with  pure 
water,  it  was  dried  in  a  preliminary  way  by  centrifugal  settling  in  platinum 
crucibles,  and  then  by  being  heated  in  an  electric  oven  at  about  130°  C.  The 
salt  was  powdered  in  an  agate  mortar  before  the  final  heating  in  a  quartz  tube 
or  platinum  boat,  as  explained  later.  It  was  shown  by  tests  with  diphenyl- 
amine  that  the  arsenate  could  be  washed  free  from  nitrates. 

Although  one  of  the  hydrogens  of  arsenic  acid  resembles  the  hydrogen  of 
strong  acids  in  its  dissociating  tendency,  the  other  two  hydrogens  are  those 
of  weak  acids.^  Hence  perceptible  hydrolysis  takes  place  in  solutions  of  salts 
of  this  acid,  even  when  the  base  is  strong,  that  of  the  tertiary  salts  being  of 
course  greatest  in  extent.  It  is  not  an  easy  matter  to  predict  the  effect  of  this 
hydrolysis  upon  the  composition  of  a  precipitate  of  silver  arsenate;  for  while 
the  Phase  Rule  allows  the  existence  of  only  one  solid  in  equilibrium  with  the 
arsenate  solution  except  at  certain  fixed  concentrations,  the  possibility  of  the 
occlusion  of  either  basic  or  acid  arsenates  by  the  silver  arsenate  still  exists. 
Experiments  only  are  able  to  throw  light  on  this  point.  Accordingly  arsenate 
solutions  of  different  conditions  of  acidity  and  alkalinity  were  used  in  the 
precipitations,  and  the  compositions  of  the  different  precipitates  were 
compared. 

Sample  A.  Commercial  C.  P.  disodium  arsenate  was  recrystallized  four 
times,  all  but  the  first  crystallization  being  conducted  in  platinum  vessels.  The 
mother-liquor  from  the  fourth  crystallization,  after  the  removal  of  the  arsenic 
by  hydrogen  sulphide,  gave  no  test  for  phosphate.  The  calculated  amount  of 
redistilled  ammonia  to  make  disodium  ammonium  arsenate  was  added  to  a 
solution  of  the  purified  salt  before  the  precipitation  of  the  silver  arsenate.  Dur- 
ing this  precipitation  the  mother-liquor  remains  essentially  neutral. 

Sample  B.  This  sample  was  made  from  disodium  arsenate  which  had  been 
recrystallized  five  times  in  platinum  vessels.    Silver  arsenate  was  precipitated 

*  Washburn  calculates  from  Walden's  conductivity  measurements  the  constant  for  the 
first  hydrogen  of  arsenic  acid  to  be  4.8X  10-'.  Jour.  Amer.  Chem.  Soc,  30,  35  (1908).  The  con- 
stants for  the  second  and  third  hydrogens  are  probably  lower  than  those  of  phosphoric  acid, 
2.0  X  10-^  and  3.6  X  10-".    Abbott  and  Bray:  Ibid.,  31,  755  (1909). 


A  REVISION   OF   THE   ATOMIC  WEIGHT   OF  ARSENIC.  75 

with  a  solution  of  this  salt  without  the  addition  of  ammonia.  Here  the  mother- 
liquor  becomes  more  and  more  acid  as  precipitation  proceeds. 

Sample  C.  Commercial  C.  P.  arsenic  trioxide  was  recrystallized  three  times 
from  dilute  hydrochloric-acid  solution,  and,  after  being  rinsed  with  water  and 
centrifugally  drained,  it  was  converted  into  arsenic  acid  by  means  of  nitric 
and  hydrochloric  acids  in  a  porcelain  dish.  The  hydrochloric  and  nitric  acids 
were  expelled  by  evaporation  nearly  to  dryness,  and  the  residue  was  twice 
evaporated  to  dryness  with  nitric  acid  in  a  platinum  dish.  After  the  residue 
had  been  dissolved  in  water,  the  solution  was  allowed  to  stand  for  some  time 
in  order  to  allow  pyro-  and  meta-arsenic  acids  to  be  converted  as  completely  as 
possible  into  ortho-arsenic  acid.  Then  sodium  carbonate,  which  had  been  twice 
crystallized  in  platinum,  was  added  to  the  solution  in  amount  sufficient  to  form 
disodium  arsenate,  and  the  product  was  crystallized  four  times  in  platinum 
vessels.  The  precipitation  of  silver  arsenate  by  adding  a  solution  of  this  salt  to 
a  solution  of  silver  nitrate  resembles  the  preparation  of  Sample  B. 

Sample  D.  A  portion  of  the  arsenic  acid  made  for  the  preparation  of 
Sample  C  was  converted  into  ammonium  dihydrogen  arsenate  by  adding  the 
calculated  amount  of  redistilled  ammonia,  and  the  salt  was  recrystallized 
five  times  in  platinum,  A  sufficient  quantity  of  ammonia  to  form  tri- 
ammonium  arsenate  was  added  to  a  solution  of  this  salt  before  the  precipi- 
tation of  the  silver  arsenate.  One  specimen  of  silver  arsenate  made  in  this 
way  was  discarded,  since  its  composition  was  very  irregular. 

Sample  E.  To  a  portion  of  the  arsenic  acid  used  for  Sample  C  recrystallized 
sodium  carbonate  was  added  in  amount  sufficient  to  form  disodium  arsenate. 
After  the  solution  had  been  evaporated  to  dryness,  the  salt  was  recrystallized 
four  times  in  platinum.  Enough  ammonia  to  form  disodium  ammonium  arse- 
nate was  added  to  a  solution  of  this  salt  before  the  precipitation  of  the  silver 
arsenate.    This  material  resembles  Sample  A. 

Sample  F.  A  portion  of  the  disodium  arsenate  prepared  for  Sample  B  was 
converted  into  trisodium  arsenate  by  means  of  recrystallized  sodium  carbonate, 
and  the  trisodium  arsenate  was  recrystallized  six  times  in  platinum  vessels. 

Sample  G.  Arsenic  trioxide  was  twice  resublimed  in  a  current  of  pure  dry  air 
and  then  once  crystallized  from  dilute  hydrochloric-acid  solution.  Next  the 
arsenious  acid  was  oxidized  to  arsenic  acid  exactly  as  described  under  Sample  C. 
Finally  the  arsenic  acid  was  converted  into  trisodium  arsenate  by  means  of 
pure  sodium  carbonate,  and  the  salt  was  crystallized  four  times  in  platinum. 
Samples  F  and  G  are  evidently  very  similar. 

In  all  the  foregoing  crystallizations  the  crystals  were  thoroughly  drained  in  a 
centrifugal  machine  employing  large  platinum  Gooch  crucibles  as  baskets,^  and 
each  crop  of  crystals  was  once  rinsed  with  a  small  quantity  of  pure  water  and 
subsequently  drained  in  the  centrifugal  machine. 

'  Baxter:  Jour.  Amer.  Chem.  Soc,  30,  286  (1908). 


76  RESEARCHES    UPON   ATOMIC   WEIGHTS. 

PURIFICATION   OF   OTHER   MATERIALS. 
SILVER  NITRATE. 

The  silver  nitrate  used  in  the  preparation  of  the  different  samples  of  silver 
arsenate  was  recrystallized  several  times  in  platinum  vessels,  with  centrifugal 
drainage,  until  the  mother-liquor  gave  no  opalescence  upon  dilution  when 
tested  in  the  nephelometer. 

HYDROBROMIC  ACID. 

One  quarter  pound  of  commercial  bromine  was  converted  into  potassium 
bromide  by  addition  to  recrystallized  potassium  oxalate.  In  the  concentrated 
solution  of  this  bromide,  in  a  distilling  flask  cooled  with  ice,  3  pounds  of 
bromine  were  dissolved  in  several  separate  portions,  each  portion  being  distilled 
from  the  solution  into  a  flask  cooled  with  ice  before  the  addition  of  the  next 
succeeding  portion.  A  portion  of  the  purified  bromine  was  then  converted 
into  potassium  bromide  with  pure  potassium  oxalate  as  before,  and  the  re- 
mainder of  the  bromine  was  distilled  in  small  portions  from  solution  in  this 
pure  potassium  bromide.  The  product  obtained  was  thus  twice  distilled  from 
a  bromide,  the  bromide  in  the  second  distillation  being  essentially  free  from 
chlorine.  This  treatment  has  already  been  proved  sufficient  to  free  bromine 
from  chlorine.^ 

Hydrobromic  acid  was  synthesized  from  the  pure  bromine  by  bubbling  hy- 
drogen gas  (made  by  the  action  of  water  on  "hydrone")  through  the  bromine 
warmed  to  40°  —  44°,  and  passing  the  mixed  gases  over  hot  platinized  asbestos 
in  a  glass  tube.  The  apparatus  was  constructed  wholly  of  glass.  The  hydrogen 
was  cleansed  by  being  passed  through  two  wash  bottles  containing  dilute  sul- 
phuric acid,  and  through  a  tower  filled  with  beads  also  moistened  with  dilute 
sulphuric  acid.  The  hydrobromic-acid  gas  was  absorbed  in  pure  water  contained 
in  a  cooled  flask.  In  order  to  remove  iodine  the  solution  of  hydrobromic  acid 
was  diluted  with  water  and  twice  boiled  with  a  small  quantity  of  free  bromine. 
Then  a  small  quantity  of  recrystallized  potassium  permanganate  was  added 
to  the  hydrobromic-acid  solution,  and  the  bromine  set  free  was  expelled  by 
boiling.  Finally,  the  acid  was  distilled  with  the  use  of  a  quartz  condenser,  the 
first  third  being  rejected.  It  was  preserved  in  a  bottle  of  Nonsol  glass  pro- 
vided with  a  ground-glass  stopper. 

The  purification  of  the  hydrobromic  acid  was  carried  on  in  conjimction  with 
Dr.  Grinnell  Jones,  who  was  engaged  in  a  parallel  research  upon  the  atomic 
weight  of  phosphorus.  Using  this  acid,  he  found  that  10.48627  gm.  of  silver 
bromide  were  obtained  from  6.02386  gm.  of  the  purest  silver.  This  ratio  of 
silver  bromide  to  silver,  100.0000  to  57.4452,  is  in  close  agreement  with  the 
most  probable  value,  100.0000  to  57.4453.^ 

^  Baxter:  Proc.Amer.  Acad. ,42,  201  {igo6);  Jour.  Amer.Chem.Soc,  28,1322;  Zeit.anorg. 
Chem.,  50,  389.    (See  page  59.)  *  Baxter:  Loc.  ciU 


A  REVISION   OF   THE  ATOMIC  WEIGHT   OF  ARSENIC.  77 

A  solution  of  hydrochloric  acid  was  purified  by  disrillation  after  dilution. 

Hydrochloric-acid  gas  was  generated  by  dropping  C.  P.  concentrated  sul- 
phuric acid  into  C.  P.  concentrated  hydrochloric  acid.  The  acids  were  shown 
to  be  essentially  free  from  arsenic. 

All  the  water  used  in  the  research  was  purified  by  distilling  the  ordinary  dis- 
tilled water  of  the  laboratory,  once  with  alkaline  permanganate  and  then  once 
alone,  in  both  cases  with  the  use  of  block-tin  condensers  which  required  no 
cork  or  rubber  connections  to  the  distilHng  flasks. 

Quartz  or  platinum  vessels  were  always  employed  in  place  of  glass,  whenever 
glass  was  unsuitable. 

METHODS   OF   ANALYSIS. 

The  first  method  of  analysis  employed  was  that  of  converting  the  silver  arse- 
nate into  silver  chloride  by  heating  in  a  current  of  hydrochloric-acid  gas.  Since 
this  process  does  not  involve  transfer  of  material  it  should  be  capable  of  giving 
results  of  great  accuracy.  Glass  and  porcelain  are  unsuitable  for  containing 
the  arsenate  during  this  process  on  account  of  the  certainty  of  their  being  at- 
tacked. The  first  attempts  at  using  quartz  for  the  purpose  resulted  in  slight 
etching  of  the  surface  of  the  tube  where  it  came  in  contact  with  the  salt.  Ex- 
perience showed,  however,  that  with  careful  manipulation  the  attacking  of  the 
quartz  could  be  wholly  prevented.  The  vessel  used  to  contain  the  arsenate  was 
a  quartz  tube  nearly  2  cm.  in  diameter  but  joined  to  small  tubes  at  each  end. 
These  tubes  were  exactly  like  those  employed  by  Richards  and  Jones  in  the 
conversion  of  silver  sulphate  into  silver  chloride.^  After  the  tube  had  been 
weighed  by  substitution  for  a  counterpoise  similar  in  shape  and  size,  a  suitable 
quantity  of  silver  arsenate  was  introduced,  and  the  tube  and  contents  were 
heated  in  a  current  of  pure  dry  air  for  between  7  and  8  hours  at  250°  C.  Al- 
though this  treatment  is  not  sufficient  to  expel  last  traces  of  moisture,  it  was 
hoped  that  by  uniform  treatment  of  the  arsenate  in  all  the  analyses  the  propor- 
tion of  water  retained  by  the  salt  could  be  reduced  to  a  constant  percentage, 
which  could  be  determined  in  separate  experiments. 

The  complete  drying  of  the  salt  by  fusion  was  not  permissible  because  of  de- 
composition of  the  arsenate  at  temperatures  in  the  neighborhood  of  its  fusing 
point.  During  the  drying  of  the  arsenate  the  quartz  tube  was  surrounded  with 
a  cylinder  of  thin  platinum  foil  and  was  contained  in  a  hard-glass  tube  connected 
with  an  apparatus  for  furnishing  a  current  of  pure  dry  air.  The  hard-glass  tube 
was  heated  by  means  of  two  aluminum  blocks  15  cm.  by  13  cm.  by  5  cm.,  one 
placed  above  the  other,  the  upper  surface  of  the  lower  block  and  the  lower  sur- 
face of  the  upper  being  suitably  grooved  to  contain  the  tube.  The  blocks  were 
bored  to  contain  a  thermometer,  the  bulb  of  which  was  located  near  the  middle 
of  the  tube.   This  oven  (fig.  4)  could  be  readily  maintained  at  constant  temper- 

^  Pub.  Car.  Inst.,  No.  69,  69(1907);  Jour.Amsr.Chem.Soc.,2g,Ss3;  Zeit.  anorg.  Chem., 
55,  80. 


78 


RESEARCHES   UPON  ATOMIC 


Fig.  4.  —  Solid  aluminum  drying  oven. 


ature  within  a  very  few  degrees  by  means  of  a  Bunsen  flame.  We  are  indebted 
to  Dr.  Arthur  Stabler  of  the  University  of  Berlin  for  the  suggestion  of  this 
method  of  heating. 

In  order  to  purify  and  dry  the  air  it  was  passed  through  a  tower  filled  with 
beads  moistened  with  dilute  silver-nitrate  solution,  through  a  tower  filled 
with  small  lumps  of  solid  potassium  hydroxide,  then  through  3  towers  filled 

with  beads  moistened  with 
concentrated  sulphuric  acid, 
and  finally  through  a  tube 
filled  with  resubUmed  phos- 
phorus pentoxide. 

The  apparatus  was  con- 
structed wholly  of  glass,  with 
ground  joints,  and  was  simi- 
lar to  that  shown  in  fig.  i, 
page  8. 

After  being  heated,  the 
quartz  tube  was  transferred 
to  a  desiccator  and  was  allowed  to  come  to  the  temperature  of  the  balance 
case  before  being  weighed.  The  quartz  tube  was  then  placed  upon  hard- 
glass  supports,  in  a  horizontal  position,  one  end  being  slipped  into  a  larger 
tube,  through  which  could  be  passed  a  current  of  either  dry  hydrochloric- 
acid  gas  or  dry  air.  The  other  end  of  the  quartz  tube  slipped  into  one  of 
the  arms  of  a  large  U-tube  filled  with  glass  pearls,  which  served  to  con- 
dense any  silver-chloride  vapor,  which  might  escape  from  the  quartz  tube. 
The  other  arm  of  the  U-tube  was  connected  with  the  flue  of  a  hood,  the 
suction  thus  caused  being  sufficient  to  prevent  the  escape  of  gaseous  arsenic 
compounds  from  the  apparatus.  The  quartz  tube  was  protected  from  dust 
by  a  covering  of  sheet  mica.       ^ 

The  usual  method  of  procedure  was  as  follows:  The  quartz  tube  containing 
the  silver  arsenate  being  in  place,  a  current  of  hydrochloric-acid  gas  was  passed 
through  the  tube,  and  the  tube  was  slowly  revolved  with  pincers  tipped  with 
platinum  wire  in  order  that  the  salt  might  be  thoroughly  exposed  to  the  action 
of  the  acid.  Neglect  to  do  this  at  the  commencement  of  the  reaction  always 
resulted  in  the  caking  of  the  salt  in  the  tube,  thereby  rendering  the  action  of 
the  acid  less  rapid.  The  hydrochloric  acid  was  dried  by  passing  through  three 
towers  containing  beads  moistened  with  concentrated  sulphuric  acid.  The  ap- 
paratus for  generating  and  purifying  the  acid  was  constructed  wholly  of  glass, 
and  was  similar  to  that  shown  in  fig.  i,  page  8. 

In  the  earlier  experiments  the  salt  was  gently  heated  from  the  commence- 
ment of  the  reaction.  To  all  outward  appearance  it  was  entirely  converted  into 
silver  chloride  in  a  few  hours.  Upon  fusion,  however,  it  presented  a  very 
cloudy  appearance,  owing  to  the  presence  of  arsenic  compounds,  which  could 


A  REVISION  OF   THE   ATOMIC   WEIGHT   OF  ARSENIC.  79 

not  be  completely  removed  even  by  keeping  the  silver  chloride  fused  in  the  cur- 
rent of  hydrochloric  acid  for  as  long  as  8  hours.  This  is  the  cause  of  the  larger 
quantities  of  arsenic  found  in  the  chloride  obtained  in  the  earlier  analyses. 
Furthermore,  the  longer  period  of  heating  at  a  temperature  above  the  fusing 
point  of  silver  chloride  accoimts  for  the  larger  amounts  of  volatilized  silver 
chloride  found  in  these  experiments. 

As  experience  was  gained,  it  was  found  best  to  expose  the  salt  first  in  the  cold 
for  about  8  hours  to  the  action  of  the  hydrochloric-acid  gas,  next  to  heat  the 
salt  gently  below  its  fusing  point  for  from  10  to  15  hours,  and  finally  to  keep  it 
barely  fused  for  from  5  to  10  hours  longer.  When  the  reaction  was  apparently 
at  an  end,  the  current  of  hydrochloric-acid  gas  was  stopped,  and  dry  air  was 
passed  through  the  tube  for  about  15  minutes  in  order  to  eUminate  hydro- 
chloric acid.  The  silver  chloride  was  then  allowed  to  solidify  in  a  uniform  thin 
layer  around  the  inside  of  the  quartz  tube  by  slowly  revolving  the  tube  during 
solidification.  The  platinum  wire  used  in  weighing  the  tube  was  slipped  on,  the 
tube  was  transferred  to  its  desiccator,  and  after  standing  several  hours  beside 
the  balance  it  was  weighed. 

In  order  to  make  sure  that  the  reaction  was  complete  the  silver  chloride  was 
again  fused,  and  exposed  to  the  action  of  hydrochloric  acid  for  several  hours 
longer.  As  a  rule,  no  change  in  weight  was  observed.  In  all  cases  constant 
weight  was  obtained  upon  heating  in  the  same  way  for  a  third  time. 

After  making  certain  that  only  a  small  quantity  of  arsenic,  if  any,  remained 
in  the  silver  chloride,  the  contents  of  the  quartz  tube  were  dissolved  in  ammonia, 
and  the  silver  chloride  was  reprecipitated  by  boiling  the  solution  to  expel  the 
ammonia  and  adding  a  small  quantity  of  sulphuric  acid.  The  solution,  after 
evaporation,  was  added  to  a  Berzelius-Marsh  apparatus  containing  arsenic-free 
zinc  and  sulphuric  acid,  and  a  mirror  of  arsenic  was  deposited  in  a  hard-glass 
capillary  tube  in  the  usual  way.  The  hydrogen  was  dried  by  calcium  chloride 
before  passing  into  the  hard-glass  tube,  and  the  generating  flask  was  cooled  with 
water  to  prevent  the  evolution  of  hydrogen  sulphide. 

The  arsenic  mirror  formed  was  compared  with  a  photograph  of  standard 
arsenic  mirrors,^  the  original  mirrors  showing  that  comparison  with  the  photo- 
graph was  equally  satisfactory.  The  correction  was  applied  by  assuming  that 
the  arsenic  was  present  in  the  silver  chloride  as  arsenic  trichloride,  although 
if  present  as  silver  arsenate  the  correction  would  be  much  smaller.  In  any 
case  the  correction  for  residual  arsenic  is  so  small  as  to  be  almost  without  effect 
upon  the  final  result. 

Ebaugh  used  essentially  the  same  method  of  heating  the  arsenate  in  hydro- 
chloric acid,  although  the  periods  were  shorter,  so  that  it  is  probable  that  the 
small  quantities  of  arsenate  used  (scarcely  over  one  gram  in  any  analysis)  did 
not  retain  weighable  amounts  of  arsenic. 

*  Sanger:  Proc.  Amer.  Acad.,  26,  24  (1891). 


8o  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

The  U-tube  used  for  collecting  volatilized  silver  chloride  was  washed  out 
thoroughly  with  dilute  ammonia,  and  the  solution  was  made  up  to  definite  vol- 
ume after  nearly  all  the  ammonia  had  been  expelled  by  boiling.  The  silver 
content  of  the  solution  was  then  compared  in  the  nephelometer  with  that  of 
standard  solutions  of  silver,  care  being  taken  that  the  tubes  were  treated  in  as 
nearly  as  possible  the  same  way. 

The  second  method  of  analysis  consisted  in  heating  the  silver  arsenate  in  a 
platinum  boat  and,  after  weighing,  dissolving  the  arsenate  in  nitric  acid  and 
precipitating  the  silver  as  chloride  or  bromide.  The  platinum  boat  was  heated 
in  a  hard-glass  tube  forming  part  of  a  bottling  apparatus,^  and  was  thus  trans- 
ferred without  exposure  to  moist  air  to  the  weighing-bottle  in  which  it  was 
always  weighed.  The  boat  and  bottle  were  weighed  by  substitution  by  com- 
parison with  a  counterpoise  similar  both  in  shape  and  volume. 

After  the  silver  arsenate  had  been  weighed,  the  boat  with  its  contents  was 
transferred  to  a  flask,  and  the  salt  was  dissolved  in  warm  5N  nitric  acid.  The 
weighing-bottle  was  rinsed  with  acid,  and  the  rinsings  were  added  to  the  main 
solution;  then  the  solution  was  carefully  transferred  to  a  large  glass-stoppered 
precipitating  flask  and  diluted  to  a  volume  of  about  i  liter. 

From  the  weight  of  silver  arsenate  the  amount  of  either  hydrochloric  or 
hydrobromic  acid  necessary  to  precipitate  the  silver  was  calculated.  A 
slight  excess  of  one  acid  or  the  other  was  then  diluted  to  about  600  c.c,  and  the 
solution  was  slowly  poured  into  the  solution  of  silver  arsenate  in  the  precipitat- 
ing flask.  After  a  few  moments'  shaking  the  precipitate  was  allowed  to  stand 
for  several  days,  with  occasional  agitation. 

The  precipitated  silver  chloride  or  silver  bromide  was  next  collected  upon  a 
weighed  Gooch  crucible,  after  it  had  been  washed  by  decantation  about  ten  times 
with  dilute  hydrochloric  acid  in  the  case  of  silver  chloride,  with  water  in  the 
case  of  silver  bromide.  After  several  hours'  heating  in  an  electric  air  bath  at 
150°  C,  and  about  2  hours'  heating  at  200°  C,  the  precipitate  was  cooled  in  a 
desiccator  and  weighed. 

In  order  to  determine  the  moisture  retained  by  the  precipitate  it  was  trans- 
ferred as  completely  as  possible  to  a  small  porcelain  crucible  and  weighed.  Then 
the  salt  was  fused  by  heating  the  small  covered  crucible  contained  in  a  large 
crucible  and  was  again  weighed. 

The  asbestos  mechanically  detached  from  the  Gooch  crucible,  together  with 
a  small  quantity  of  silver  chloride  or  silver  bromide  which  escaped  the  crucible, 
was  collected  upon  a  small  filter  through  which  the  filtrate  and  wash-waters 
were  passed,  and  the  filter  paper  was  ignited  in  a  small  weighed  porcelain  cru- 
cible. Before  being  weighed  the  ash  was  treated  with  a  drop  of  nitric  and  a 
drop  of  either  hydrochloric  or  hydrobromic  acid  and  again  heated. 

^  See  page  9. 


A  REVISION   OF   THE   ATOMIC   WEIGHT   OF  ARSENIC. 


8l 


The  filtrate  and  wash- waters  were  evaporated  to  small  bulk.  The  precipitat- 
ing flask  was  rinsed  with  ammonia,  and  the  rinsing  was  added  to  the  evapo- 
rated filtrate  and  wash-water.  Then  the  solution  was  diluted  to  definite 
volimae,  and  the  silver  content  was  determined  by  comparison  with  standard 
silver  solutions  in  the  nephelometer. 

The  operations  of  precipitating  and  collecting  the  silver  halides  were  all  car- 
ried out  in  a  room  lighted  only  with  ruby  light. 


INSOLUBLE   RESIDUE. 

All  the  specimens  of  silver  arsenate,  after  being  heated  at  250°  C,  when  dis- 
solved in  dilute  nitric  acid,  were  found  to  contain  a  small  amount  of  insoluble 
residue,  which  would  dissolve  only  in  rather  concentrated  nitric  acid.  Although 
the  proportion  of  this  residue  was  apparently  increased  by  exposure  to  light, 
specimens  of  the  arsenate  which  had  been  prepared  wholly  in  the  dark  room 
were  not  free  from  it.  No  process  of  purification  to  which  the  soluble  arsenate 
used  in  the  preparation  of  the  silver  arsenate  was  subjected  seemed  to  have  the 
slightest  effect  upon  the  proportion  of  insoluble  matter.  A  similar  phenomenon 
was  met  by  Dr.  Grinnell  Jones  in  the  preparation  of  silver  phosphate  (page  178). 

Although  the  amount  of  this  residue  in  one  gram  of  silver  arsenate  which 
had  been  treated  as  in  the  analyses  for  silver  was  not  over  0.00005  gm.,  it  was 
important  to  determine  its  silver  content.  This  was  done  in  three  cases  in 
which  the  proportion  of  residue  had  been  purposely  increased  as  much  as  pos- 
sible by  exposure  to  hght.  The  arsenate  was  dissolved  in  dilute  nitric  acid, 
and  the  residue  was  collected  upon  a  weighed  platinum  Gooch  crucible,  the 
detached  asbestos  shreds  being  carefully  determined  by  filtration  upon  a  filter 
paper.  The  weight  of  residue  was  found  by  reweighing  the  crucible.  After 
the  residue  had  been  dissolved  in  concentrated  nitric  acid  and  the  solution  had 
been  diluted  to  definite  volume,  the  silver  content  of  the  solution  was  ascer- 
tained by  comparison  with  standard  silver  solutions  in  a  nephelometer. 


Weight  of 

silver 
arsenate. 

Weight  of 
insoluble 
residue. 

Weight  of 
silver. 

Per  cent  of 
silver. 

gm. 

4.26 

10.00 

9.28 

gm. 
0.00198 
0.00228 
0.00657 

gm. 
0.00143 
0.00162 
0.00500 

72.3 
71.I 
76.1 

Average 7^.2 

Theoretical  per  cent  of  silver  in  silver  arsenate    .   .   .    70.0 

The  first  of  the  above  determinations  was  made  with  a  sample  of  silver  arse- 
nate which  had  been  exposed  to  bright  light  inside  a  desiccator  for  a  month. 


82  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

During  this  time  the  quartz  tube  containing  the  salt  showed  no  perceptmie 
change  in  weight.  The  third  determination  also  was  made  with  a  sample  of 
salt  which  had  been  exposed  to  bright  light  for  3  weeks  in  a  dry  state.  In  the 
second  determination  the  salt  was  exposed  to  light  under  water  for  one  week. 
Two  facts  show  that  the  presence  of  the  small  proportion  of  the  residue  in  the 
arsenate  could  have  had  no  important  efifect  upon  the  results.  In  the  first  place, 
the  formation  of  the  insoluble  matter  under  the  influence  of  light  is  not  attended 
by  change  in  weight.  In  the  second  place,  the  silver  content  of  the  residue  is 
very  near  that  of  silver  arsenate.  Nevertheless,  care  was  taken  to  protect  the 
arsenate  as  far  as  possible  from  exposure  to  light. 

DETERMINATION   OF  MOISTURE   IN   DRIED   SILVER   ARSENATE. 

T.  W.  Richards  ^  and  others  have  already  drawn  attention  to  the  fact  that  it 
is  not  possible,  without  fusion,  to  dry  completely  a  substance  formed  in  aqueous 
solution,  owing  to  the  mechanical  retention  of  liquid  in  pockets  within  the 
solid.  In  the  case  of  silver  arsenate,  although  it  is  possible  to  fuse  the  salt,  the 
temperature  necessary  is  so  high  that  decomposition  of  the  salt  takes  place  to 
some  extent.  Hence  the  loss  in  weight  on  fusion  can  not  be  used  as  a  true 
measure  of  the  water  content  of  the  salt.  Since  decomposition  of  the  salt  could 
produce  only  easily  condensible  substances  and  oxygen,  the  difficulty  was  over- 
come in  the  present  instance  by  fusing  weighed  quantities  of  the  salt  in  a  current 
of  pure  dry  air  and  collecting  the  water  vapor  in  a  weighed  phosphorus  pentox- 
ide  tube.  Of  course  great  pains  were  taken  to  treat  the  salt  used  in  the  water 
determinations  in  exactly  the  same  way  as  that  used  in  the  analyses  for  silver. 

The  procedure  was  as  follows:  A  sample  of  salt  very  nearly  as  pure  as  that 
used  in  the  silver  analyses  was  weighed  out  in  a  copper  boat  which  had  been 
previously  cleaned  and  ignited  in  the  blast  lamp  to  remove  organic  matter. 
The  boat  was  placed  in  a  hard-glass  tube  and  was  heated  for  between  7  and  8 
hoiu-s  at  250°  C,  in  a  current  of  dry  air.  In  these  experiments,  before  passing 
through  the  drjdng  towers  the  air  had  first  been  passed  over  hot  copper  oxide 
in  order  to  oxidize  any  organic  matter  it  might  contain.  Furthermore,  the  con- 
centrated sulphuric  acid  in  the  drying  towers  had  been  heated  with  a  small 
quantity  of  potassium  dichromate.  One  end  of  the  hard-glass  tube  was  con- 
nected to  the  apparatus  for  suppl3dng  pure  air,  by  means  of  a  well-fitting  ground 
joint  upon  which  no  lubricant  was  used.  The  other  end  was  sealed  to  a  small 
hard-glass  tube  which  was  surrounded  with  a  damp  cloth  during  the  fusion  of 
the  salt  in  order  to  facilitate  condensation  of  any  silver  or  arsenic  compounds 
vaporized  from  the  salt.  As  a  matter  of  fact,  very  little  sublimation  actually 
took  place. 

In  order  to  fuse  silver  arsenate  within  the  hard-glass  tube  it  was  necessary 
to  use  the  hottest  flame  of  the  blast  lamp,  the  tube  being  covered  with  a  semi- 

*  Zeit.  physik.  Chetn.,  46, 194  (1903). 


REVISION   OF   THE   ATOMIC  WEIGHT   OF   ARSENIC. 


83 


cylinder  of  sheet  iron.  Furthermore,  since  at  this  temperature  even  the  hard 
glass  became  very  soft,  it  was  found  necessary  to  wrap  the  tube  with  asbestos 
and  closely  wound  iron  wire  for  several  inches  at  the  piont  where  fusion  took 
place.  This  also  served  to  distribute  the  heat  more  evenly  and  to  prevent  the 
tube  from  cracking  during  the  experiment. 

Just  before  the  salt  was  fused  a  carefully  weighed  U-tube  containing  resub- 
limed  phosphorus  pentoxide  was  attached  to  the  end  of  the  tube,  and  beyond 
this  was  joined  another  similar  tube  which  served  as  a  protection  against  any 
moisture  which  might  diffuse  back  into  the  weighed  tube  from  the  outside  air. 
These  phosphorus  pentoxide  tubes  were  provided  with  one  way  ground  glass 
stopcocks  lubricated  with  Ramsay  desiccator  grease. 

The  salt  was  heated  for  25  minutes  with  the  hottest  flame  of  the  blast  lamp, 
being  then  completely  fused,  and  was  further  heated  for  35  minutes  at  a  con- 
siderably lower  temperature  in  order  to  make  certain  that  all  moisture  was  car- 
ried into  the  absorption  tube  by  the  current  of  air.  Finally  the  phosphorus 
pentoxide  tube  was  reweighed. 

The  pentoxide  tube  was  weighed  by  substitution  with  the  use  of  a  counter- 
poise of  the  same  size  and  weight  filled  with  glass  beads.  Before  being  weighed 
both  tubes  were  carefully  wiped  with  a  damp  cloth  and  were  allowed  to  stand 
near  the  balance  case  for  one  hour.  One  stopcock  in  each  tube  was  opened  im- 
mediately before  the  tube  was  hung  upon  the  balance,  in  order  to  insure  equi- 
librium between  the  internal  and  external  air  pressure.  The  stopcock  of  the 
counterpoise  was  left  open  during  the  weighing.  Owing  to  the  considerable 
length  of  time  required  for  the  tubes  to  come  to  equilibrium  on  the  balance,  it 
was  considered  unsafe  to  leave  the  stopcock  of  the  pentoxide  tube  open  during 
the  weighing.  As  a  check  on  the  first  weight  of  the  pentoxide  tube  one  stopcock 
was  opened  and  closed  and  its  weight  determined  a  second  time.  Ordinarily  no 
change  in  weight  was  observed. 


Weight  of 
silver  arsenate. 

Weight  of 
water. 

Weight  of  water 
per  gram  of  salt. 

gm. 
11.09 
13-59 
17-23 
12.57 

gm. 
0.00083 
0.00073 
0.00085 
0.00057 

gm. 
0.000075 
0.000054 
0.000049 
0.000045 

Average 0.000056 

Since  it  seemed  possible  that  the  hard-glass  tube  itself,  when  heated  nearly 
to  fusion,  might  give  off  traces  of  water  vapor,  two  blank  determinations  were 
made  by  heating  the  empty  hard-glass  tube  in  exactly  the  same  way  as  in  the 
water  determinations.  These  determinations  showed  a  gain  in  weight  of  the 
pentoxide  tube  of  0.00022  and  0.00037  gm.  respectively,  the  average  being 


84 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


0.00030  gm.  This  correction  was  confirmed  in  another  experiment  in  which 
the  hard-glass  tube  was  kept  at  the  highest  temperature  obtainable  with  the 
blast  lamp  for  an  hour.  The  observed  gain  in  weight  of  the  absorption  tube 
was  0.00048  gm.  A  negative  correction  of  0.00030  gm.  was  applied  in  each  water 
determination. 

In  order  to  allow  for  moisture  the  weight  of  the  arsenate  was  therefore  always 
corrected  by  subtracting  0.000056  gm.  per  gram  of  salt.  Ebaugh  took  no  notice 
of  the  water  contained  in  silver  arsenate  which  had  been  dried  at  only  170°. 

SPECIFIC   GRAVITY   OF   SILVER   ARSENATE. 

In  order  that  the  apparent  weight  of  the  silver  arsenate  might  be  corrected 
to  a  vacuum  standard,  the  specific  gravity  of  the  arsenate  was  found  by  deter- 
mining the  weight  of  toluol  displaced  by  a  known  quantity  of  salt.  The  toluol 
was  first  dried  by  means  of  stick  soda  and  was  then  distilled,  with  rejection  of 
the  first  portion  of  the  distillate.  Its  specific  gravity  at  25"  referred  to  water 
at  4°  was  found  to  be  0.8620.  Pains  were  taken  to  remove  air  from  the  arsenate 
when  covered  with  toluol  by  placing  the  pycnometer  in  an  exhausted  desiccator. 


Weight  of 

silver  arsenate 

in  vacuum. 

Weight  of 

displaced  toluol 

in  vacuum. 

Specific  gravity 

of  silver 
arsenate 
25°/4°. 

gm. 
S.1690 
5.6729 

gm. 
0.6688 
0.7350 

gm. 

6.662 

6.652 

Average 6.657 

The  following  vacuum  corrections  were  applied: 


Specific 
gravity. 

Vacuum 
correction. 

Weights 

Toluol 

Silver  arsenate     .... 

Silver  chloride 

Silver  bromide     .... 

8.3 

0.862 

6.657 

5.56 

6.473 

+  0.00126 
+  0.000036 
+  0.000071 
+  0.000041 

All  weighings  were  made  upon  a  nearly  new  short-armed  Troemner  balance, 
easily  sensitive  to  0.02  mg.  with  a  load  of  50  gm. 

The  gold-plated  Sartorius  weights  were  several  times  carefully  standardized 
to  hundredths  of  a  milligram  by  the  method  described  by  Richards,^  and  were 
used  for  no  other  work. 


*  Jour.  Amer.  Ckem.,  Soc,  22,  144  (1900). 


A  REVISION   OF   THE  ATOMIC  WEIGHT   OF  ARSENIC. 


85 


RESULTS   AND   DISCUSSION. 
Series  I.    sAgCl:  Ag3As04. 


No. 

of 

analysis. 

Sample 
AgsAsOj. 

Corrected 

weight  of 

AggAsOi 

in  vacuum. 

Weight 

of 

AgCl 

in  vacuum. 

Residual 
A8CI3. 

Volatilized 
AgCl. 

Corrected 
weight  of 

AgCl 
in  vacuum. 

Ratio 

3  AgCl: 

AgsAtOi. 

I 

2 

3 

4 
5 

A 
A 
A 
B 
C 

gm. 
3-17276 
2.65042 
3.51128 
5.83614 
5-72252 

gm. 
2.94912 
2.46364 

3-26395 
5.42499 

S-31947 

gm. 
0.00004 
0.0CX)04 

O.CX>OOI 

0.00001 
0.00001 

gm. 
0.00014 
0.00007 
0.00002 
0.00005 
0.00001 

gm. 
2.94922 

2.46367 
3.26396 

5-42503 
S-31947 

0.929544 
0-929539 
0.929564 
0.929558 
0.929568 

Avera 

Z6        ... 

0  0  20  c  c  c 

Series  II.    3 AgCl:  AgsAsO^. 


No. 

of 
anal- 
ysis. 


Sample 

of 
AgsAsO^ 


Corrected 
weight  of 
AgsAsOi 


Weight 

of 

AgCl 


Weight 

of 
asbestos. 


Dissolved 

AgCl 

from 

filtrate. 


Loss 

on 

fusion. 


Corrected 

weight  of 

AgCl 


Ratio 
3  AgCl: 
AgjAsOa. 


gm. 

4-59149 
3.38270 


gm. 
4.26815 
3.14401 


gm. 

0.CXK)08 

0.00037 


gm. 

0.<X>0I2 

0.00013 


gm. 
o.o<X)39 
0.00015 


gm. 
4.26796 
3-14436 


0-929537 
0-929542 


Average ,■    •    •. 0.929540 

Average  of  all  analyses  in  Series  I  and  II 0.929550 

Per  cent  of  Ag  in  Ag3As04 69.9609  * 


Series  III .    3 AgBr :  Ags AsO^. 


No. 
of 
anal- 
ysis. 

Corrected 

Weight 

Dissolved 
AgBr 

Corrected 

Sample 

weight  of 

of 

Weight 

Loss 

weight,  of 

Ratio 

of 

Ag3As04 

AgBr 

of 

on 

AgBr 

3  AgBr: 

AggAsOi 

m 

m 

asbestos. 

fusion. 

in 

Ag3As04. 

vacuum. 

vacuum. 

vacuum. 

gm. 

gm. 

gm. 

gm. 

gm. 

gm. 

8 

c 

^■isis-^ 

10.66581 

0.00008 

0.00004 

0.00040 

10.66553 

I.21787 

9 

D 

6.76988 

8.24529 

0.00024 

O.OCXX37 

O.CX30I5 

8.24545 

1. 21796 

10 

D 

5.19424 

6.32569 

0.00017 

0.00009 

O.OCX305 

6.32590 

1. 21787 

II 

D 

5-33914 

6.50251 

0.00009 

O.OOCX36 

0.00008 

6.50258 

I.21791 

12 

E 

8.24054 

10.03497 

0.00053 

0.00014 

o.oooi  2 

10.03552 

1. 21782 

13 

E 

7-57962 

9-23134 

O.CKX)2I 

0.00005 

0.00013 

9.23147 

I.21793 

14 

E 

6.05230 

7-37066 

0.00038 

0.00005 

0.00003 

7-37106 

1. 21789 

Average 
Per  cent 
Average 

I.21789 
69.9622  2 
69.9616 

of  Ag  in  Ag3As04    . 

per  cent  of  Ag  in  AgsJ 

\S04.     .     . 

*  Ag:  AgCl  =  0.752632:1.000000. 
2  Ag:  AgBr  =  0.574453:  i.oooooo. 


Richards  and  Wells:  Loc.  cit. 
Baxter:  Loc.  cit. 


86 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 
Series  IV.    3 AgCl:  Ag3As04. 


No. 

of 

analysis. 

Sample 

of 
AgsAsOi. 

Corrected 
weight  of 

AggAsOl 

in 
vacuum. 

Weight  of 
AgCl  in 
vacuum. 

Residual 

ASCI3. 

Volatized 
AgCl. 

Corrected 

weight  of 

AgCl  in 

vacuum. 

Ratio 

3  AgCl: 

AgsAsOi. 

gm. 

gm. 

gm. 

gm. 

gm. 

IS 

F 

4.67268 

4-34393 

0.00006 

0.00002 

4-34389 

0.929636 

16 

F 

7.71882 

7.17602 

0.00007 

0.00002 

7-I7S97 

0.929672 

17 

G 

5.28049 

4.90908 

0.0000 1 

O.OOOOI 

4.90908 

0.929664 

18 

G 

4-25346 

3-9S422 

0.00000 

0.00002 

3-95424 

0.929652 

19 

G 

3-47340 

3.22892 

0.00000 

O.OOOOI 

3-22893 

0.929616 

20 

G 

5-17269 

4.80877 

0.00000 

0.00002 

4.80879 

0.929650 

21 

G 

4.10766 

3.81856 

0.00000 

0.00002 

3-81858 

0.929624 

Average 

0.02064^ 

Series  V.    3AgCl:  Ag3As04. 


No. 
of 
anal- 
ysis. 

Sample 

AgsAsOi. 

Corrected 
weight  of 

AggAsOi 

in 
vacuum. 

Weight 

AgCl 

in 

vacuum. 

Weight 

of 

asbestos. 

Dis- 
solved 
AgCl 
from 
filtrate. 

Loss 

on 

fusion. 

Corrected 

weight  of 

AgCl 

in 

vacuum. 

Ratio 
3  AgCl: 
AgsAsO,. 

22 

G 

gm,. 
5-47133 

gm. 
5.08686 

gm. 
0.00009 

gm. 
0.00014 

gm. 
0.00066 

gm. 
5.08643 

0.929652 

Average  of  all  analvses  in  Series  IV  an 

dV      .    .    . 

0.929646 

69.9681 

Per  rent  of  Aor  in 

Ag3As04 

Series  VI.    3AgBr:  Ag3As04. 


No. 
of 
analy- 
sis. 


23 
24 
25 
26 


Sample 

of 
AgaAsO,. 


Corrected 
weight  of 
AgsAsOi 


gm. 
4.96261 

5-31743 
4.46882 
4.16702 


Weight 

of 
AgBr  in 
vacuum. 


gm. 
6.04438 
6.47645 
5-44273 
S-07533 


Weight 

of 
asbestos. 


gm. 
0.00004 
0.00015 
0.00026 
o.oooio 


Dissolved 

AgBr 

from 

filtrate. 


gm. 
0.00010 
0.00009 
O.oooi  I 
0.00004 


Loss 

on 

fusion. 


gm. 
0.00012 
0.0001 1 
0.00010 
0.00008 


Corrected 

weight  of 

AgBr 


gm. 
6.04440 
6.47658 
5-44300 
5-07539 


Ratio 

3  AgBr: 

AgsAsOi. 


1. 217988 
1. 217991 
1. 217995 
1.217990 


Average     i. 217991 

Per  cent  of  Ag  in|Ag3As04 69.9678 

Average  per  cent  of  Ag  in  Ag3As04 69.9680 


The  first  point  to  be  noted  in  the  foregoing  tables  is  that  the  results  can  be 
divided  into  two  distinct  groups  according  to  the  samples  of  arsenate  employed, 
Series  I,  II,  and  III,  with  Samples  A  to  E,  giving  values  for  the  per  cent  of 
silver  in  the  arsenate  lower  than  Series  IV,  V,  and  VI,  with  Samples  F  and  G. 


A   REVISION   OF    THE   ATOMIC   WEIGHT    OF   ARSENIC.  87 

In  the  second  place,  both  methods  for  determining  the  ratio  of  the  arsenate 
to  the  chloride  give  essentially  identical  results.  This  is  shown  by  the  agreement 
of  Series  I  and  II,  and  that  of  Series  IV  and  V. 

Finally,  the  per  cent  of  silver  in  silver  arsenate  found  in  Series  I  and  II  agrees 
within  less  than  0.002  per  cent  with  that  found  in  Series  III.  This  agreement, 
together  with  that  of  the  individual  analyses  of  each  series,  indicates  both  uni- 
formity in  the  material  employed  and  purity  of  the  hydrochloric  and  hydro- 
bromic  acids,  as  well  as  accuracy  in  the  analytical  work.  The  agreement  of 
Series  IV  and  V  with  Series  VI  is  closer  still. 

In  the  following  table  are  given  the  sources  of  the  various  samples  of 
silver  arsenate: 


Sample  A  . 

.  Na2NH4As04 

Sample  E  . 

.  Na2NH4As04 

Sample  B  . 

.  Na2HAs04 

Sample  F  . 

.  NasAsOi 

Sample  C  . 

.  NajHAsOi 

Sample  G  . 

.  Na3As04 

Sample  D  . 

.    (NH4)3AS04 

Since  in  the  precipitation  of  the  silver  arsenate  the  arsenate  solution  was 
always  added  to  the  silver-nitrate  solution,  the  mother-liquor  was  in  every  case 
either  neutral  or  acid,  but  in  no  case  permanently  alkaline.  Still,  local  accu- 
mulations of  arsenate  undoubtedly  exist  for  short  periods,  and  precipitates 
formed  at  these  points  may  be  affected  by  the  conditions  existing  in  the  arsenate 
solution.  In  the  case  of  Samples  F  and  G  the  arsenate  solution  is  decidedly 
alkaline,  owing  to  hydrolysis,  but  less  so  with  Samples  A,  D,  and  E,  and  least 
with  Samples  B  and  C.  In  the  case  of  Samples  B  and  C,  acid  accumulates  in  the 
solution  during  the  precipitation  of  the  silver  arsenate.  In  comparing  the  results 
from  the  different  samples  of  silver  arsenate  it  must  be  noted  that  occluded 
basic  salt  would  increase  the  apparent  percentage  of  silver  in  the  arsenate.  In 
the  case  of  Samples  F  and  G  the  conditions  are  most  favorable  for  the  occlusion 
of  basic  salts,  and  these  two  samples  actually  yield  a  higher  percetttage  of  silver 
than  the  other  samples.  On  the  other  hand  accumulation  of  acid  in  the  solution 
in  which  the  precipitation  of  the  silver  arsenate  is  taking  place  is  found  to  have 
no  tendency  to  promote  occlusion  of  acid  salts,  since  Samples  B  and  C  give 
results  agreeing  closely  with  those  of  Samples  A,  D,  and  E.  These  two  facts 
lead  to  the  conclusion  that  Samples  A  to  E  represent  normal  trisilver  arsenate, 
and  that  Samples  F  and  G  contain  basic  impurities. 

In  order  to  calculate  the  atomic  weight  of  arsenic  from  the  per  cent  of  silver 
in  silver  arsenate  a  knowledge  of  the  ratio  of  the  atomic  weights  of  silver  and 
oxygen  is  necessary.  A  slight  uncertainty  exists  as  to  this  ratio,  hence  calcula- 
tions have  been  made  upon  the  basis  of  two  extreme  values  for  silver,  107.880 
and  107.870  .  This  has  been  done  only  with  the  results  of  Series  I,  II,  and  III, 
since,  as  has  been  pointed  out,  they  are  probably  nearer  the  truth  than  those  of 
Series  IV,  V,  and  VI.  The  difference  between  the  two  sets  of  results  amounts 
only  to  0.06  per  cent  in  the  atomic  weight  of  arsenic. 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


Series  I  and  II. 

Series  III. 

If  Ag  =  107.880,    As  = 
If  Ag  =  107.870,    As  = 

74.961 
74.948 

74-953 
74-940 

When  the  results  of  Series  I  and  II  are  averaged  with  those  of  Series  III,  it 
is  found  that 

If  Ag  =  107.880  As  =  74.957 

If  Ag  =  107.870  As  =  74.944 

The  atomic  weight  of  arsenic  is  being  subjected  to  further  investigation  in 
this  laboratory,  by  determination  of  the  ratio  of  arsenic  trioxide  to  iodine. 

The  most  important  results  of  this  research  may  be  briefly  summed  up  as 
follows: 

1.  Methods  for  the  preparation  of  normal  trisilver  arsenate  were  devised. 

2.  It  is  shown  that  trisilver  arsenate  precipitated  by  means  of  trisodium 
arsenate  probably  contains  occluded  basic  impurity. 

3.  It  is  shown  that  silver  arsenate  can  not  be  completely  dried  without  fusion. 

4.  The  specific  gravity  of  unfused  trisilver  arsenate  is  found  to  be  6.66 
at  25°  C,  referred  to  water  at  4°  C. 

5.  The  per  cent  of  silver  in  silver  arsenate  is  found  to  be  69.9616  by  three 
closely  agreeing  methods. 

6.  With  two  assumed  values  for  the  atomic  weight  of  silver  referred  to 
oxygen  16.000,  the  atomic  weight  of  arsenic  is  found  to  have  the  following  values; 


If  Ag  =  107.88 
If  Ag  =  107.87 


As  =  74.96 
As  =  74.94 


VIL 

A    REVISION    OF   THE   ATOMIC    WEIGHT    OF 

IODINE. 

THE   SYNTHESIS   OF  SILVER  IODIDE  AND   THE  RATIO 

OF  SIL  VER   IODIDE  TO  SIL  VER  BROMIDE 

AND  SILVER  CHLORIDE. 


By  Gregory  Paul  Baxter. 


Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  40,  419  (1904)  ;  41,  73  (1905). 

Journal  of  the  American  Chemical  Society,  26,  1577  (1904)  ;  27,  876  (1905). 

Zeitschrift  fiir  anorganische  Chemie,  43,  14  (1905) ;  46,  36  (1905). 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A   REVISION  OF  THE  ATOMIC  WEIGHT  OF  IODINE. 

THE   SYNTHESIS   OF  SILVER   IODIDE   AND  THE  RATIO   OF 
SILVER    IODIDE   TO    SILVER    BROMIDE 
AND   SILVER    CHLORIDE. 


INTRODUCTION. 


The  atomic  weight  of  iodine  was  for  some  time  considered  one  of  the  best  de- 
termined of  chemical  constants,  owing  to  the  extremely  concordant  results  of  Stas 
and  Marignac,  who  both  deduced  the  value  126.79  (O  =  16.000  and  Ag= 
107.880)  from  syntheses  of  silver  iodide.  Recently,  however,  two  series  of 
determinations,  one  by  Ladenburg  and  one  by  Scott,  have  yielded  results  over 
0.1  higher  than  the  above.  Ladenburg's  method  consisted  in  heating  silver 
iodide  in  a  current  of  chlorine  until  all  the  iodine  had  been  displaced,  and  gave 
the  result  126.90,  while  Scott,  like  Stas  and  Marignac,  synthesized  silver  iodide 
and  obtained  in  two  analyses  the  values  126.90  and  126.92.^  The  anomaly  in  the 
atomic  weights  of  iodine  and  tellurium  has  always  been  of  the  greatest  in- 
terest, and  has  led  to  a  large  number  of  investigations  upon  the  atomic  weight 
of  tellurium  during  the  last  few  years,  which  have  shown  conclusively  that  the 
value  of  this  constant  lies  in  the  neighborhood  of  127.6.  The  doubt  thrown  upon 
the  atomic  weight  of  iodine  by  the  work  of  Ladenburg  and  Scott  made  it 
imperative  to  subject  iodine  to  further  careful  investigation. 

In  choosing  a  compound  for  an  atomic  weight  determination,  two  of  the  most 
important  points  to  be  considered  are  the  stability  of  the  substance  under  vari- 
ous conditions,  and  the  certainty  with  which  the  atomic  weights  are  known  of 
those  elements,  besides  the  one  under  examination,  which  compose  it.  One 
of  the  few  compounds  of  iodine  which  satisfy  both  the  above  conditions  is  silver 
iodide.  Accordingly  synthesis  of  silver  iodide  from  a  weighed  amount  of  silver 
was  selected  for  the  first  method  of  investigation. 

^  The  higher  values  of  Scott  and  Ladenburg  were  confirmed,  shortly  before  the  publication 
of  the  first  portion  of  this  paper,  by  Kothner  and  Aeuer,  in  a  preliminary  notice  of  experiments 
involving  syntheses  of  silver  iodide  as  well  as  a  repetition  of  Ladenburg's  work,  from  which  they 
conclude  that  the  atomic  weight  of  iodine  can  not  be  lower  than  126.90.  Details  of  their  work 
are  not  given.  See  Ber.  d.  d.  chem.  Gesell.,  37,  2536  (1904).  A  critical  discussion  of  all  the 
earlier  work  upon  the  atomic  weight  of  iodine  is  to  be  found  at  the  end  of  this  paper. 

91 


92  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

RATIO    OF   SILVER   TO   SILVER   IODIDE. 
PURIFICATION   OF   MATERIALS. 

SILVER. 

This  substance  was  purified,  by  methods  which  have  been  described  previ- 
ously in  detail/  by  twice  precipitating  silver  chloride  from  strongly  acid  solu- 
tion, with  subsequent  reduction  by  means  of  invert  sugar  and  sodium  hydroxide 
in  each  case,  especially  purified  reagents  being  employed  in  the  second  precipi- 
tation and  reduction.  The  product,  after  fusion  before  a  blowpipe  upon  a  cru- 
cible lined  with  the  purest  lime,  was  converted  into  electrolytic  crystals  and 
these  were  fused  in  a  current  of  hydrogen  upon  a  boat  of  pure  lime.  The  re- 
sulting buttons  of  metal,  after  being  freed  from  adhering  lime  by  treatment  with 
dilute  acid,  were  cut  into  fragments  of  from  3  to  5  gm.  by  means  of  a  clean 
chisel  and  anvil,  and  the  iron  adhering  to  the  surface  was  removed  by  warm- 
ing the  pieces  repeatedly  with  fresh  portions  of  dilute  nitric  acid,  until  iron 
could  no  longer  be  detected  in  the  acid.  The  fragments  were  washed  thoroughly 
with  the  purest  water,  then  with  ammonia,  and  again  with  water  as  before. 
They  were  allowed  to  dry  in  the  air,  and  finally  the  last  traces  of  moisture  were 
eliminated  by  heating  in  a  vacuum,  since  silver  which  is  heated  to  a  high  tem- 
perature in  the  air  has  been  shown  by  Stas  to  absorb  considerable  oxygen.'^ 
Silver  prepared  in  a  similar  fashion  by  investigators  in  this  laboratory  has  always 
been  found  to  be  pure,  and  since  two  different  samples  were  used  in  the  following 
work  with  identical  results,  there  can  be  no  doubt  of  the  purity  of  that  employed 
here.  Furthermore,  one  synthesis  of  silver  chloride,  carried  out  with  a  portion 
of  this  silver  by  Wells  in  the  investigation  of  Richards  and  Wells  upon  the 
atomic  weight  of  chlorine,  yielded  a  value  which  is  identical  with  the  average 
of  other  similar  determinations  made  with  his  own  silver.^ 

IODINE. 

The  chief  impurities  in  commercial  iodine  are  halogens  of  lower  atomic 
weight  and  iodide  of  cyanogen.  These  were  removed  by  dissolving  iodine  in  a 
strong  solution  of  half  its  weight  of  potassic  iodide,  and  distilling  the  greater 
portion  of  the  iodine  from  a  retort  into  a  flask  cooled  with  cold  water.  The 
iodine  thus  obtained  was  next  converted  into  hydriodic  acid  by  covering  it  with 
considetable  water  and  passing  through  the  solution  a  stream  of  hydrogen  sul- 
phide. This  gas  was  generated  by  the  action  of  dilute  sulphuric  acid  upon 
ferrous  sulphide,  and  was  purified  by  bubbling  through  three  gas-washing 
bottles  containing  water  and  through  two  towers  filled  with  beads  moistened 
with  water.   The  reaction  between  the  hydrogen  sulphide  and  the  iodine  results 

1  See  pages  6  and  22. 

2  CEuvres  Completes,  3, 125. 

3  Pub.  Car.  Inst.,  No.  28,  p.  65  (1905);  Jour.  Amer.  Chem.  Sac,  27,  524;  Zeit.  anorg. 
Chetn.,  47,  130. 


A  REVISION  OF   THE  ATOMIC  WEIGHT   OF   IODINE.  93 

chiefly  in  the  formation  of  hydriodic  acid  and  the  precipitation  of  sulphur, 
although  a  small  quantity  of  sulphuric  acid  is  always  produced.  Iodide  of 
cyanogen  with  hydrogen  sulphide  jdelds  hydrocyanic  acid,  hydriodic  acid,  and 
sulphur.^  The  solution  was  first  boiled  for  a  short  time  until  the  sulphur  had 
clotted  together,  and  the  sulphur  was  removed  by  filtration.  The  clear  solution 
was  then  boiled  for  several  hours  in  order  to  eliminate  the  hydrocyanic  acid.^ 
Finally  the  hydriodic  acid  was  partially  converted  into  iodine  by  distilling  the 
solution  with  a  sUght  excess  of  potassium  permanganate.  This  permanganate 
must  also  have  had  the  effect  of  oxidizing  any  organic  impurities.  Since  in  the 
reaction  three-eighths  of  the  iodine  remain  in  the  form  of  iodides,  the  resulting 
iodine  was  thus  subjected  to  a  second  distillation  from  an  iodide.  The  product 
was  again  converted  into  hydriodic  acid  by  means  of  hydrogen  sulphide,  and 
this  again  into  iodine  by  recrystallized  potassium  permanganate  which  was  free 
from  even  a  trace  of  chlorine.  Since  in  the  case  of  bromine  it  has  already  been 
shown  that  two  distillations  from  a  pure  bromide  are  sufficient  to  free  this  ele- 
ment from  chlorine,^  three  distillations  of  iodine  from  a  solution  of  an  iodide 
should  insure  complete  elimination  of  chlorine  and  bromine  even  if  the  original 
substance  were  very  impure.  The  purified  iodine  was  again  converted  into  hy- 
driodic acid  by  hydrogen  sulphide,  and  this  into  ammonimn  iodide  by  an  excess 
of  freshly  distilled  ammonia.  This  material  was  used  in  the  analyses  made  with 
Sample  I. 

It  was  suggested  to  me  by  Prof.  T.  W.  Richards  that  an  undiscovered  element 
resembling  the  halogens,  but  of  a  higher  atomic  weight,  might  exist,  and  an 
attempt  to  detect  the  existence  of  such  an  element  was  carried  out  as  follows: 
One  pound  of  iodine  was  purified  as  described  above,  but  the  process  of  convert- 
ing the  iodine  into  hydriodic  acid  by  hydrogen  sulphide,  and  the  hydriodic  acid 
into  iodine  by  potassic  permanganate,  was  repeated  four  times.  The  iodine 
obtained  in  the  last  distillation  with  permanganate,  amounting  in  all  to  about 
50  gm.,  was  set  free  in  4  fractions  by  introducing  the  permanganate  in  portions 
of  about  2  gm.  each.  An  element  belonging  to  the  halogen  family,  and  of  higher 
atomic  weight  than  iodine,  should  be  more  easily  set  free  than  iodine,  hence  the 
first  of  the  4  fractions  should  have  contained  nearly  all  of  such  an  element 
which  occurred  in  the  pound  of  iodine.  Each  one  of  the  fractions  was  once  dis- 
tilled with  pure  water,  and  was  then  converted  into  ammonium  iodide,  as  in  the 
case  of  Sample  I.  The  first  fraction  was  fraction  i  of  Sample  II,  the  fourth 
was  fraction  4  of  Sample  II.  No  difference  in  appearance  between  these  samples 
and  Sample  I  or  even  the  original  crude  iodine  could  be  detected. 

All  reagents  used  in  the  course  of  the  purification  or  in  the  analyses  were 
carefully  purified.   Acids  and  ammonia  were  redistilled,  usually  with  the  use  of 

^  Dammer,  Handb.  d.  anorg.  Chem.,  II,  i,  432. 

*  Richards  and  Singer,  Amer.  Chem.  Jour.,  27,  205  (1902). 

'  See  page  60. 


94  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

a  platinum  condenser.  The  water  was  distilled  twice,  once  from  alkaline  per- 
manganate, once  from  a  solution  containing  a  trace  of  sulphuric  acid.  In  both 
distillations  block  tin  condensers  were  used. 

METHOD    OF   SYNTHESIS. 

In  order  to  convert  a  known  weight  of  silver  into  silver  iodide  essentially 
the  same  method  was  used  as  that  employed  in  the  research  upon  the  synthesis 
of  silver  bromide  (page  57).  The  silver  was  dissolved  slowly  in  redistilled 
nitric  acid  which  had  been  diluted  with  an  equal  volume  of  water  in  the  flask 
described  on  page  12.  The  solution  was  diluted  and  was  heated  until  all 
nitrous  acid  was  destroyed  and  all  oxides  of  nitrogen  were  expelled.  It  was 
then  transferred  to  a  glass-stoppered  precipitating-flask  and  after  dilution 
until  not  more  than  i  gm.  of  silver  was  contained  in  100  c.c,  an  excess  of  am- 
monia was  added  and  then  a  slight  excess  of  the  solution  of  pure  ammonium 
iodide,  which  also  had  been  diluted  until  not  more  concentrated  than  i  per 
cent.  The  flask  was  stoppered  and  shaken  for  some  time,  and  was  allowed  to 
stand  for  about  a  day.  Next  a  very  slight  excess  of  nitric  acid  was  introduced, 
the  flask  was  again  shaken  for  some  time,  and  was  allowed  to  stand  for  one  or 
two  days  until  the  solution  above  the  precipitate  seemed  perfectly  clear.  The 
precipitate  was  washed  by  decantation  with  i  per  cent  nitric  acid  at  least  eight 
times,  and  was  transferred  with  water  to  a  weighed  Gooch  crucible  through 
which  the  washings  had  been  poured.  Finally  the  crucible  was  heated  in  an 
electric  air-bath  for  several  hours,  first  at  100°  to  110°,  then  at  200°  C. 

Although  silver  iodide  is  very  slightly  sensitive  to  diffused  daylight,  the 
operations  of  precipitation  and  filtration  were  performed  in  a  large  cupboard 
lighted  with  orange  light.  When  taken  out  of  the  dark  room,  the  flask  was 
enveloped  in  several  thicknesses  of  black  cloth. 

The  tendency  of  precipitated  silver  iodide  to  pass  into  a  colloidal  condition 
when  washed  with  pure  water  is  very  marked.  It  has  been  found  by  Stas  and 
others  that  if  the  water  is  previously  heated  to  about  60°  C-,  and  plenty  of  time 
is  allowed  for  the  precipitate  to  settle  at  that  temperature,  this  tendency  can  be 
overcome.  When  washed  with  warm  water,  however,  silver  iodide  adheres  to 
the  flask  to  such  an  extent  that  it  is  impossible  entirely  to  remove  the  precipi- 
tate even  by  vigorous  rubbing. 

The  conditions  most  favorable  for  preserving  the  precipitate  in  such  a  state 
that  it  could  be  easily  and  rapidly  manipulated  were  as  follows.  In  the  first 
place  precipitation  was  carried  out  in  ammoniacal  solution,  because  the  silver 
iodide  clotted  together  much  more  rapidly  than  in  acid  solution,  owing  prob- 
ably to  the  greater  solubility  of  silver  iodide  in  the  ammoniacal  solution.  A 
large  excess  of  nitric  acid  also  coagulated  the  precipitate,  but  since  the  acid 
caused  the  separation  of  a  large  amount  of  iodine,  it  could  not  be  employed  in 
the  present  case.    Dilute  nitric  acid,  on  the  other  hand,  proved  to  be  a  satis- 


A   REVISION    OF    THE   ATOMIC   WEIGHT    OF   IODINE.  95 

factory  medium  for  washing  the  silver  iodide,  for  it  was  found  that  the  precipi- 
tate could  be  washed  indefinitely  with  even  i  per  cent  nitric  acid  without  passing 
into  the  colloidal  state.  The  washings  obtained  in  this  way,  when  treated  with 
an  iodide,  gave  no  trace  of  opalescence  even  on  long  standing,  showing  that 
silver  iodide  is  essentially  insoluble  in  the  dilute  acid.  When  silver  iodide  which 
has  been  washed  with  nitric  acid  only  is  dried  at  a  high  temperature,  a  loss  of 
iodine  takes  place,  owing  to  action  on  the  silver  iodide  by  the  nitric  acid.  This 
was  evident  from  the  dark  color  of  the  iodide.  In  order  to  avoid  this  difficulty, 
the  precipitate  was  finally  transferred  to  the  Gooch  crucible  with  pure  water, 
sometimes  after  one  rinsing  with  water.  The  nitric  acid  was  so  completely  dis- 
placed by  this  means  that  the  precipitate  did  not  darken  even  on  fusion.  Un- 
fortunately, even  this  slight  washing  with  water  in  most  cases  caused  some  of  the 
precipitate  to  pass  through  the  crucible  in  a  colloidal  condition,  consequently 
these  last  washings  were  collected  separately. 

In  the  earlier  analyses  the  silver  iodide  in  this  colloidal  solution  was  deter- 
mined by  first  boiling  the  solution  with  a  small  quantity  of  ammonium  iodide 
until  the  silver  iodide  was  coagulated,  and  then  collecting  the  precipitate  upon 
a  small  filter  paper,  together  with  the  asbestos  shreds  and  any  silver  iodide 
which  were  contained  in  the  remainder  of  the  filtrate  and  washings.  As  a  rule 
this  operation  was  performed  only  after  the  wash-waters  had  stood  for  2  or  3 
days,  in  order  that  insoluble  matter  might  have  time  to  settle.  The  bottom  of 
each  flask  was  rubbed  gently  with  a  rubber-tipped  stirring-rod  to  detach  adher- 
ing particles.  Finally  the  filter  paper  was  burned  in  a  weighed  porcelain  crucible 
at  as  low  a  temperature  as  possible,  for  if  a  high  temperature  is  employed,  a  loss 
of  silver  iodide  by  volatilization  occurs.  The  ash  was  treated  with  a  drop  of 
nitric  acid,  and,  after  warming,  a  drop  of  ammonium  iodide  solution  was  added. 
The  excess  of  acid  and  ammoniiuii  salt  was  driven  off  and  the  crucible  was 
weighed. 

In  these  earlier  analyses  it  was  found  necessary  to  detach  small  particles  of 
adhering  silver  iodide  from  the  neck  of  the  precipitating  flask  by  rubbing  with 
a  rubber-tipped  rod.  Later  a  better  method  was  devised  for  collecting  this  small 
amount  of  precipitate  together  with  that  contained  in  the  colloidal  washings. 
First  the  flask  was  rinsed  with  a  small  quantity  of  a  solution  of  potassium 
cyanide,  and  this  solution  was  poured  into  the  colloidal  washings.  Then  the 
solution  was  evaporated  to  small  bulk  and  was  electrolyzed  in  a  weighed  plat- 
inum crucible,  which  was  heated  to  130°  in  an  electric  oven  and  was  finally 
reweighed.  The  film  was  dissolved  in  dilute  nitric  acid  and  the  solution  was 
precipitated  with  an  excess  of  ammonium  iodide.  This  precipitate  of  silver 
iodide,  if  over  0.0005  gm.  in  weight,  was  collected  on  a  small  weighed  Gooch 
crucible,  and  the  filtrate,  together  with  the  original  filtrate  and  wash -waters, 
was  passed  through  a  small  filter  repeatedly  until  clear.  If  the  precipitate 
weighed  less  than  the  above  quantity,  it  was  collected  wholly  upon  a  filter. 
These  filters  were,  of  course,  ignited  and  treated  as  before.    The  weight  of  the 


96 


RESEARCHES   UPON  ATOMIC   WEIGHTS. 


silver  iodide  and  asbestos  was  used  in  computing  the  results,  the  weight  of  elec- 
trolyzed  silver  serving  merely  as  a  check  upon  the  weight  of  silver  iodide. 

In  order  to  drive  off  the  last  traces  of  moisture  from  the  silver  iodide  it  was 
fused  in  a  porcelain  crucible.  The  bulk  of  the  precipitate,  freed  as  completely 
as  possible  from  asbestos,  was  transferred  to  a  clean  crucible,  which  was  then 
weighed  with  its  cover.  The  crucible  and  cover  were  placed  inside  a  large  porce- 
lain crucible,  and  were  heated  until  fusion  took  place.  A  temperature  much 
above  the  melting  point  of  silver  iodide  was  avoided,  since  this  substance  is 
distinctly  volatile  at  higher  temperatures.  The  loss  in  weight  was  then  deter- 
mined. The  fused  salt  when  cold  was  Hght  yellow  in  color,  with  no  trace  of 
darkening,  showing  that  no  appreciable  loss  of  iodine  had  taken  place. 


SPECIFIC    GRAVITY   OF    SILVER    IODIDE. 

The  specific  gravity  of  pure  silver  iodide  was  found  by  displacement  of  water. 
Three  determinations  were  made  with  material  which  had  been  fused  in  a  por- 
celain crucible  and  cooled  by  pouring  the  fused  mass  upon  a  cold  tile.  The 
sohdified  salt  was  broken  into  small  fragments  and  was  introduced  into  a 
weighed  pycnometer  for  solids,  which  was  then  reweighed.  Water  sufficient  to 
cover  the  salt  was  introduced  into  the  pycnometer  and  the  system  was  placed 
in  a  vacuum  desiccator  which  was  kept  exhausted  imtil  it  seemed  probable  that 
all  air  had  been  extracted  from  the  solid.  Then  the  pycnometer  was  immersed 
in  a  bath  at  25°  C.  and  exactly  filled  with  water.  Finally  the  system  was 
weighed.  Two  more  determinations  were  carried  out  with  material  which  had 
been  cast  into  sticks  by  pouring  the  fused  salt  into  a  porcelain  boat.    These 


Weight  of  silver  iodide 
in  vacuum. 

Weight  of  water  dis- 
placed in  vacuum. 

Density  of  silver 
iodide 

250/40. 

gm. 
15-5871 
14.514I 
16.3685 

gm. 
2.7590 
2.5652 
2.8853 

5-650 
5-658 
S-673 

Average  .    .    .    5.660 

21.5876 
16.7274 

3.8017 
2.950s 

5.678 
5-669 

Average  .    .    .    5.674^ 

The  values  for  the  density  of  silver  iodide  found  by  other  experimenters  are  as  follows : 

BouUay,  Ann.  de  Chitn.  et  de  Phys.,  (2)  43,  266  (1830) S-614 

KsiTsten,  Jour,  fur  Chem.  und  Phys. ,6s,  4x7  {18^2) 5.026 

FiVaol,  Ann.  de  Chim.  et  de  Phys.,  (s)  21,  4iy  {1847) S-Soo 

Schiff,  Ann.  der  Chem.  und  Pharm.,  108,  21  (1858) 5.35 

H.  St.  Clair  Deville,  Comptes  Rendus,  64,  325  (1867) S-687  at  0° 

Schroeder,  Ann.  der  Chem.  und  Pharm.,  192,  295  (1878) S-650  to  5.718 


A  REVISION   OF   THE  ATOMIC  WEIGHT   OF  IODINE. 


97 


sticks  were  used  in  an  elongated  fonn  of  pycnometer  for  solids.^  The  average 
of  the  last  two  experiments  is  undoubtedly  more  accurate  than  that  of  the  first 
three,  since  it  was  extremely  difficult  to  piunp  out  the  air  from  the  more  porous 
material  used  in  the  first  experiments. 

The  vacuum  correction  for  silver  iodide,  when  weighed  with  brass  weights, 
calculated  from  this  value  for  its  specific  gravity,  is  +0.000067  gm.  for  every 
apparent  gram  of  salt.^  A  vacuum  correction  of  —0.000031  gm.  was  applied 
to  every  apparent  gram  of  silver. 

The  Atomic  Weight  of  Iodine. 


Series  I.    Ag:AgI. 


No. 
of 
anal- 
ysis. 


Sam- 
pleof 

io- 
dine. 


Weight  of 
silver  in 
vacuum. 


gm. 
5-23123 

3-57039 
4.60798 
4.52467 
4.66256 


Weight  of 

silver 
iodide  in 
vacuum. 


gm. 
11.38074 

7.77023 
10.02747 

9.84834 
IO.14601 


Loss 

on 

fusion. 


gm. 
0.00030 

not  fused 
0.00047 
0.00036 
0.00062 


Weight  of 

silver 

from 

filtrate. 


gm. 


Weight  of 

silver 
iodide  and 
asbestos 
fr.  filtrate. 


gm. 
0.00482 
0.00007 
o.ooioo 
0.00020 
0.00048 


Corrected 
weight 
of  silver 
iodide  in 
vacuum. 


gm. 
11.38526 

7.77030 
10.02800 

9.84818 
10.14587 


Ratio 
Ag:AgI 


0.459474 
0.459492 

0.459511 
0.459448 

0.459552 


Atomic 

weight  of 

iodine. 

Ag  = 

107.880. 


126.910 
126.901 
126.891 
126.925 
126.870 


Average 0.459495   126.899 


Series  11.    Ag:AgI. 


6 

I 

4.77244 

10.38664 

0.00020 

0.00033 

0.00050 

10.38694 

0.459464 

7 

1 

4.82882 

10.50872 

0.00035 

0,00053 

0.00140 

10.50977 

0.459460 

8 

11,1 

4.04262 

8.79714 

0.00058 

0.00095 

8.79751 

0.459519 

9 

11,1 

1.64711 

3-58502 

0.00020 

0.00015 

0.00032 

3-58514 

0.459427 

10 

11,2 

4.86054 

10.57436 

0.00122 

0.0025 

lost. 

10.57314 

0.459470 

II 

11,2 

4.83482 

10.52209 

0.00044 

0.00032 

0.00072 

10.52237 

0.459480 

12 

11,3 

4.97120 

10.81692 

0.00025 

0.00052 

0.00129 

10.81796 

0.459532 

13 

11,3 

3.53858 

7.70136 

0.00038 

0.00017 

0.00035 

7-70133 

0.459476 

14 

11,3 

3.89693 

8.48178 

0.00026 

0.00012 

0.00032 

8.48184 

0.459444 

15 

11,4 

5.33031 

11.59780 

0.00034 

0.00185 

0.00360 

1 1. 60106 

0.459467 

16 

11,4 

5.08748 

11.07227 

0.00035 

0.00017 

0.00063 

11.07255 

0.459468 

126.915 

126.917 

126.887 
126.934 

126.912 
126.907 

126.881 

126.909 
126.926 
126.914 
126.913 


Average 0.459473  126,910 

Average,  rejecting  Analysis  12 0.459468  126.913 

Average  of  analyses  with  Sample  I 0.459462  126.916 

Average  of  analyses  with  Sample  II,  Fraction  i     .   .   .  0.459473  1 26.911 

"               "                    "                  "        2    .   .   .  0,459475  126,911 

"               "                    "                  "       3     .   .   .  0,459460  126.918 

"               "                    "                  "        4     .   .   .  0,459468  126,914 


1  Baxter  and  Hines:  Amer.  Chem.  Jour.,  31,  222  (1904). 

2  In  the  original  publication  of  this  paper  a  slightly  higher  vacuum  correction  was  em- 
ployed owing  to  an  incorrect  assumption  as  to  the  specific  gravity  of  the  weights,  which  are 
here  taken  at  8.3, 


9  8  RESEARCHES   UPON  ATOMIC  WEIGHTS. 

The  platinum-plated  brass  weights  were  twice  carefully  standardized  to  hun- 
dredths of  a  milligram,  and  the  two  sets  of  corrections  agreed  to  within  two 
hundredths  of  a  milligram  in  every  case.  Two  short-armed  Troemner  balances, 
sensitive  to  0.02  mg.  with  a  load  of  50  gm.  were  used  in  the  work.  All  weighings 
were  made  by  substitution.  In  the  case  of  the  Gooch  crucible  a  platinum  tare, 
and  in  the  case  of  the  porcelain  crucibles,  porcelain  tares  were  used. 

In  the  preceding  table  are  given  the  results  of  all  the  syntheses  which  were 
completed  without  accident. 

The  results  tabulated  in  Series  I  were  undoubtedly  less  accurate  than  those  of 
Series  II,  since  it  was  in  the  earlier  analyses  that  experience  in  handling  silver 
iodide  was  gained.  Hence  these  analyses  are  collected  by  themselves.  In 
Series  II  the  rejection  of  analysis  12  seems  justifiable,  since  two  other  analyses 
made  with  the  same  material  yielded  results  in  close  agreement  and  con- 
siderably higher  than  that  of  analysis  12. 

From  an  examination  of  this  table  two  important  deductions  may  be  drawn. 
First,  there  can  be  little  doubt  of  the  identity  of  the  different  samples  of  iodine; 
for  the  extreme  averages  of  the  results  from  the  various  samples  differ  by  only 
seven  thousandths  of  a  unit.  The  variation  in  the  values  from  the  analysis  of 
Sample  II,  fraction  i,  is  undoubtedly  due  in  the  case  of  analysis  8  to  lack  of 
experience  in  manipulating  sUver  iodide,  for  this  analysis  was  the  first  one  to  be 
completed  in  the  second  series,  and  in  the  case  of  analysis  9  to  the  small  quan- 
tity of  available  material.  One  may  conclude  without  hesitation  that,  in  the 
material  examined  in  this  investigation  at  least,  no  important  quantity  of  a 
new  halogen  existed. 

The  second  important  deduction  is  that  the  atomic  weight  of  iodine  is  evi- 
dently very  slightly  greater  than  126.913;  for  most  of  the  experimental  errors, 
such  as  loss  of  silver  iodide,  or  loss  of  iodine  by  the  silver  iodide,  would  have 
caused  the  result  to  be  too  low. 


A  REVISION  OF   THE  ATOMIC   WEIGHT   OF  IODINE.  99 

RATIO   OF   SILVER   TO   IODINE. 

The  ratio  of  silver  to  iodine  was  next  investigated.  The  problem  of  obtaining 
iodine  in  a  dry  state  presented  most  difficulties  in  this  portion  of  the  research, 
and  was  finally  solved  as  follows:  Iodine,  which  had  been  prepared  in  the  same 
way  as  Sample  I,  was  first  freed  from  the  greater  part  of  the  water  which  it  con- 
tained by  exposure  to  concentrated  sulphuric  acid,  which  had  been  boiled  to 
remove  every  trace  of  hydrochloric  acid.  It  was  then  sublimed  from  a  crystalliz- 
ing dish  to  the  bottom  of  a  glass  dish  half  filled  with  water,  which  covered  the 
crystallizing  dish.  In  this  way  the  greater  part  of  the  "included"  moisture 
must  have  been  eliminated.  The  sublimed  crystals  were  finally  again  sublimed 
in  a  current  of  pure  dry  air  from  a  porcelain  boat  contained  in  a  hard-glass  tube 
into  the  weighing  tube.  This  weighing  tube  was  about  10  cm.  long  and  15  mm. 
diameter,  and  was  drawn  down  to  less  than  half  this  diameter  at  both  ends. 
Glass  stoppers  were  groimd  into  both  ends  of  the  tube.  The  air  was  purified  and 
dried  by  passing  over  beads  moistened  with  a  solution  of  silver  nitrate,  then  over 
sodic  carbonate,  and  finally  over  3  feet  of  beads  moistened  with  concentrated 
sulphuric  acid,  all  in  an  apparatus  made  entirely  of  glass  and  connected  with 
the  sublimation  tube  by  means  of  a  ground  glass  Joint.  During  the  final  sub- 
limation of  the  iodine,  the  end  of  the  hard-glass  tube,  which  had  been  drawn 
down  to  small  diameter,  was  inserted  into  one  end  of  the  weighing  tube.  After 
the  weighing  tube  had  been  filled,  the  glass  stoppers  were  inserted,  the  tube  was 
carefully  wiped  with  a  slightly  damp  "chemically  clean"  cloth,  and  was 
allowed  to  stand  in  a  desiccator  for  some  time.  It  was  then  weighed.  Need- 
less to  say,  the  weighing  tube  had  been  originally  treated  as  above  before 
being  weighed  empty.  Both  weighings  were  made  with  a  counterpoise  exactly 
similar  to  the  weighing  tube. 

In  all  but  one  of  the  experiments  the  weighing  tube  was  broken  during  the 
solution  of  the  iodine,  so  that  it  was  impossible  to  weigh  the  tube  after  the  ex- 
periment, and  determine  how  much  the  glass  was  attacked  by  the  warm  iodine. 
In  one  experiment,  however,  a  loss  in  weight  of  0.0003  2  gm.  was  found.  Whether 
or  not  this  change  was  accompanied  by  the  evolution  of  a  gas,  the  loss  in  weight 
must  have  been  at  least  partially  due  to  combination  of  the  iodine  with  the 
alkaline  metals  of  the  glass,  with  the  formation  of  soluble  iodides  which  were 
dissolved  by  the  solution  of  sulphurous  acid.  Hence  the  error  must  have  been 
considerably  less  than  0.3  mg.,  if,  as  was  necessarily  the  case  owing  to  breakage 
of  the  tubes,  the  first  weight  of  the  tube  was  used  in  determining  the  weight  of 
the  iodine. 

Immediately  after  being  weighed,  in  order  to  avoid  loss  by  volatilization,  the 
iodine  was  converted  into  hydriodic  acid  by  means  of  pure  sulphurous  acid. 
This  acid  was  made  by  heating  sulphuric  acid  with  metallic  copper  and  collect- 
ing the  sulphurous  oxide  in  water,  and  then  distilling  the  sulphurous  oxide  from 
the  solution  into  the  purest  water.   During  this  distillation  any  trace  of  halogen 


100  RESEARCHES   UPON  ATOMIC  WEIGHTS. 

acid  which  might  have  had  its  source  in  either  copper  or  sulphuric  acid  must 
have  been  completely  eliminated,  for  it  must  have  been  almost  wholly  in  the 
ionized  condition. 

A  considerable  quantity  of  this  sulphurous  acid  was  poured  into  one  of  the 
precipitating  flasks,  and  the  weighing  tube,  containing  the  iodine,  was  intro- 
duced, after  the  lower  stopper  had  been  allowed  to  drop  out  of  the  tube  into  the 
flask.  The  sulphurous  acid  immediately  sealed  the  open  end  of  the  tube  so  that 
no  iodine  vapor  could  escape.  The  other  stopper  was  then  removed  by  means  of 
a  platinum  wire,  the  wire  was  rinsed  into  the  flask,  and  the  flask  was  quickly 
closed  by  means  of  its  glass  stopper.  Solution  of  the  iodine  in  the  sulphurous 
acid  was  hastened  by  gently  agitating  the  flask.  Any  iodine  vapor  which  es- 
caped from  the  tube  must  have  been  instantly  converted  into  hydriodic  acid. 
After  sufficient  time  had  been  allowed  for  every  trace  of  this  hydriodic  acid  to  be 
absorbed  by  the  solution,  the  flask  was  opened  and  the  solution  was  transferred 
to  another  precipitating  flask,  and  a  slight  excess  of  the  purest  ammonia  was 
added.  When  iodine  is  dissolved  in  a  large  excess  of  sulphurous  acid  the  solu- 
tion becomes  colored  yellow,  owing  to  the  formation  of  an  iodide  of  sulphur,^ 
and  upon  standing,  this  solution  may  deposit  sulphur.  But  since  the  solution 
was  made  alkaline  with  ammonia  as  soon  as  the  iodine  was  dissolved,  with 
the  complete  disappearance  of  the  color  and  without  the  formation  of  the 
slightest  trace  of  a  precipitate,  no  danger  was  to  be  feared  from  this 
source. 

From  the  weight  of  the  iodine,  the  weight  of  silver  necessary  exactly  to  com- 
bine with  it  was  calculated.  This  silver  was  weighed  out  and  dissolved  in  nitric 
acid  as  described  previously.  The  solution  was  diluted  until  not  stronger  than 
I  per  cent  and  was  added  slowly  to  the  already  dilute  solution  of  ammonic  iodide 
in  the  precipitating  flask.  The  flask  was  shaken  for  some  time,  and  was  then 
made  acid  with  a  considerable  excess  of  nitric  acid.  Long-continued  shaking, 
followed  by  several  days'  standing,  3delded  a  clear  supernatant  solution.  25  c.c. 
portions  of  this  solution  were  pipetted  into  nephelometer  tubes  and  were 
tested  with  cubic  centimeter  portions  of  hundredth  normal  silver  nitrate  and 
hydriodic  acid  solutions  in  a  nephelometer.  If  an  excess  of  either  iodine  or 
silver  was  present,  the  deficiency  of  the  other  was  made  up  in  the  remaining 
solution  by  addition  of  standard  silver  nitrate  or  hydriodic  acid  until  the  exact 
end-point  was  reached.  It  was  never  necessary  to  add  more  than  o.i  mg.  of 
either  iodine  or  silver,  so  that  the  liquid  removed  from  the  flask  for  the  tests 
could  be  neglected.  This  end-point  is  very  sharp  in  the  case  of  silver  and  pure 
iodine;  for  so  little  silver  iodide  is  dissolved  that  the  two  nephelometer  tubes 
remain  almost  absolutely  clear.  An  excess  of  o.i  mg.  of  silver  in  a  liter  of  solu- 
tion is  easily  detected.  This  almost  complete  lack  of  opalescence  in  the  nephelo- 
meter tubes  is  strong  evidence  of  the  absence  of  even  a  trace  of  either  chlorine  or 

*  Dammer  :  Handb.  d.  anorg.  Chent.,  i,  557. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF  IODINE. 


lOI 


bromine  in  the  iodine,  for  silver  chloride  or  bromide,  on  account  of  their  greater 
solubility,  would  have  produced  much  more  marked  precipitates. 

The  specific  gravity  of  soUd  iodine  is  assumed  to  be  4.933,^  hence  a  vacuum 
correction  of  +0.000099  is  applied  to  every  apparent  gram  of  iodine,  as  well  as 
one  of  —0.000031  to  every  apparent  grtim  of  silver. 


RESULTS. 

The  Atomic  Weight  of  Iodine. 


Series  III.    Ag:I. 


No.  of 
analysis. 

Weight  of 
silver  in 
vacuum. 

Weight  of 
iodine  in 
vacuum. 

Ratio. 
Ag:I. 

Atomic 

weight  of 

iodine. 

Ag=  107.880. 

17 

18 

19 

am. 

5-54444 
6.27838 
4.57992 

gm. 
6.52286 
7-38645 
5-38812 

0.850001 
0.849986 
0.850004 

126.917 
126.920 
126.917 

Average 0.849997         126.918 

As  is  to  be  expected,  the  average  of  this  series  is  slightly  higher  than  that  of 
the  previous  one,  and  undoubtedly  represents  more  closely  the  true  value  of 
the  atomic  weight  of  iodine.  Nevertheless,  the  investigation  was  not  allowed 
to  rest  at^this  point. 


^  Ladenburg:  Ber.  d.  d.  Chem.  GeselL,  35, 1256  (1902).  In  the  original  publication  of  this 
paper  a  slightly  higher  vacuum  correction  was  employed,  owing  to  an  incorrect  assumption  as 
to  the  specific  gravity  of  the  weights,  which  are  here  taken  at  8.3. 


102  RESEARCHES   UPON  ATOMIC  WEIGHTS. 

RATIO  OF   SILVER   IODIDE   TO    SILVER   CHLORIDE. 

In  any  atomic  weight  investigation  it  is  extremely  desirable  to  obtain  the 
value  sought  by  reference  to  as  many  different  well-known  atomic  weights  as 
possible.  The  method  of  heating  silver  iodide  in  a  current  of  chlorine,  which  has 
already  been  used  by  Berzelius  and  Dumas  as  well  as  by  Ladenburg,  furnishes 
the  ratio  between  silver  iodide  and  silver  chloride,  and  seemed  capable  of  yield- 
ing trustworthy  results;  for  since  the  silver  halides  fuse  at  a  comparatively  low 
temperature,  there  is  no  possibility  of  inclusion  of  silver  iodide  by  the  silver 
chlorides  formed  in  the  reaction,  and  hence  there  is  certainty  that  the  reaction 
will  be  complete. 

First,  silver  iodide  was  prepared  by  precipitating  an  ammoniacal  solution  of 
the  purest  ammoniimi  iodide  (Sample  I)  with  a  solution  of  recrystallized  silver 
nitrate.  In  this  operation  a  slight  excess  of  ammonium  iodide  was  used.  The 
precipitate  was  well  washed  with  i  per  cent  nitric  acid,  rinsed  with  water,  and 
was  collected  on  a  Gooch  crucible  with  the  use  of  a  disk  of  filter  paper  instead 
of  an  asbestos  mat.  In  this  way  contamination  of  the  precipitate  with  asbestos 
shreds  was  avoided.  The  silver  iodide  was  finally  dried  in  an  air-bath  at  about 
ioo°  C.  for  at  least  8  hours.  After  removal  of  the  filter  paper,  those  portions  of 
the  precipitate  which  had  come  in  contact  with  the  filter  paper  were  cut  away 
with  a  clean  knife.  Next  the  substance  was  fused  in  a  weighed  crucible  protected 
from  the  flame  by  a  very  large  crucible.  While  the  silver  iodide  was  fused,  a 
small  quantity  of  the  purest  iodine  was  placed  upon  the  lower  side  of  a  second 
crucible  cover,  and  this  cover  was  substituted  for  the  one  which  had  been  weighed 
with  the  crucible.  The  iodine  immediately  vaporized  and  the  silver  iodide  was 
thus  fused  in  an  atmosphere  containing  iodine  vapor.  Finally,  the  cover  was 
removed,  so  that  the  uncombined  iodine  escaped  from  the  crucible,  and  the  salt 
was  kept  fused,  covered  with  the  original  crucible  cover,  until  it  was  certain  that 
all  excess  of  iodine  had  been  eliminated.  That  no  excess  of  iodine  was  retained 
by  the  silver  iodide  was  readily  shown  in  one  experiment  with  i8  gm.  of  salt  by 
reheating  the  salt  to  its  fusing  point  and  reweighing.  A  loss  in  weight  of  only 
0.00003  gi^'  resulted. 

After  the  silver  iodide  had  been  weighed  it  was  heated  in  a  current  of  chlorine. 
This  gas  was  generated  by  dropping  concentrated  hydrochloric  acid  upon  man- 
ganese dioxide,  and  it  was  piuified  and  dried  by  bubbling  through  water  and 
passing  through  a  3-foot  tube  filled  with  beads  moistened  with  concentrated 
sulphuric  acid.  Traces  of  bromine  or  iodine  in  the  chlorine  would  have  been  no 
disadvantage,  and  it  is  inconceivable  that  it  should  have  contained  fluorine. 
In  order  to  prevent  spattering  of  the  fused  salt  from  the  crucible,  a  perforated 
porcelain  disk,  which  fitted  the  crucible  half-way  between  the  bottom  and  the 
top,  was  placed  in  the  crucible.   This  disk  was  always  weighed  with  the  crucible. 

The  chlorine  was  conducted  into  the  crucible  by  means  of  a  small  hard-glass 
tube,  which  passed  just  through  the  perforated  cover  of  a  Rose  crucible,  while 


A  REVISION  OF   THE   ATOMIC  WEIGHT  OF   IODINE.  I03 

the  apparatus  for  generating  chlorine  was  constructed  wholly  of  glass  with  the 
exception  of  the  joint  between  this  hard-glass  tube  and  the  drying  tube.  The 
chlorine  did  not  come  in  contact  with  the  rubber  tube  used  in  making  the  con- 
nection tight,  however,  for  the  hard-glass  tube  telescoped  into  the  drying  tube 
for  some  distance  and  the  joint  was  sealed  with  concentrated  sulphuric  acid. 
In  order  to  avoid  volatilization  of  the  silver  salts,  the  heat  appHed  was  very 
gentle  and  only  sufficient  to  fuse  the  mixed  haUdes.  Even  at  this  temperature 
the  iodine  was  rapidly  replaced.  Heating  in  chlorine  was  continued  some  time 
after  the  color  of  the  iodine  vapor  had  ceased  to  be  visible.  Then  the  Rose 
cover  was  replaced  by  the  ordinary  cover  and  the  silver  chloride  was  kept  fused 
for  several  minutes,  with  occasional  Kfting  of  the  cover,  so  as  to  drive  off  any 
chlorine  which  might  have  been  dissolved  by  the  fused  salt.  Although  fused 
silver  chloride,  when  cooled  in  chlorine,  dissolves  this  gas  very  appreciably,  no 
evidence  was  found  that  any  remained  in  the  solidified  salt  when  it  was  heated 
for  a  short  time  in  air.  In  one  experiment  18  gm.  of  the  chloride  after  the  usual 
treatment  were  re-fused  in  air  with  a  loss  in  weight  of  only  0.00002  gm.  The 
same  chloride,  when  fused  and  cooled  in  chlorine,  gained  5  mg. 

It  was  soon  discovered  that  a  porcelain  crucible,  when  used  for  the  conversion 
of  silver  iodide  into  silver  chloride,  gradually  gained  in  weight.  In  the  first  ex- 
periment this  gain  was  over  3  mg.,  and  in  several  subsequent  experiments  with 
the  same  crucible,  amounted  to  slightly  less  than  a  milligram  in  each  case.  This 
gain  did  not  take  place  to  the  slightest  extent  when  the  crucible  was  heated  alone 
in  chlorine,  nor  after  the  reaction  was  complete,  for  the  weight  of  the  crucible 
and  silver  chloride  very  soon  became  constant.  Probably  it  was  due  partially 
to  solution  of  the  silver  salt  in  the  glaze,  as  was  shown  by  slight  discoloration  on 
the  bottom  of  the  crucible.  Possibly,  however,  it  was  caused  by  the  attacking 
of  the  glaze  by  the  "nascent"  chlorine  and  iodine  set  free  in  the  process,  per- 
haps with  evolution  of  oxygen.  At  any  rate  too  great  an  uncertainty  existed 
as  to  the  weight  of  the  crucible  at  the  end  of  the  experiment.  Accordingly  quartz 
crucibles  were  next  employed  for  the  same  purpose.  These  crucibles  behaved 
in  an  ideal  fashion,  for  they  remained  practically  constant  in  weight  through 
the  process.  It  was  only  necessary  to  rotate  the  crucibles  during  the  solidifica- 
tion of  the  fused  salts  in  order  that  the  salt  might  solidify  in  a  thin  layer  on 
the  sides  of  the  crucible.  Neglect  to  do  this  almost  invariably  resulted  in  the 
cracking  of  the  crucible. 

After  the  first  weighing  of  the  crucible,  it  was  again  heated  in  chlorine  for  an 
hour,  and  again  cooled  and  weighed.  In  no  case  did  a  loss  in  weight  of  more 
than  0.1  mg.  take  place,  showing  both  that  the  iodine  was  completely  replaced, 
and  that  no  silver  chloride  had  volatilized.  The  following  five  experiments  were 
completed  with  quartz  crucibles  in  the  manner  described.  A  vacuum  correc- 
tion of  -f  0.000071  is  appHed  to  every  apparent  gram  of  silver  chloride.^ 

1  From  the  density  5.56  determined  by  Richards  and  StuU,  not  yet  published,  the  weights 
being  assumed  to  have  the  specific  gravity  8.3. 


I0| 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


RESULTS. 

Atomic  Weight  OF  Iodine.     Series  IV.    AgI:AgCl. 


No.  of 
analysis. 

Weight  of 
silver  iodide 
in  vacuum. 

Weight  of 

silver  chloride 

in  vacuum. 

Ratio. 
Agl  :  AgCl. 

Atomic  weight 

of  iodine. 

Ag  =  107.SS0. 

CI  =  35-457 

20 

21 

22 
23 
24 

mg. 

9.26856 

6.72058 

II.31S22 

10.07026 

13.49224 

gm. 
5.65786 
4.10257 
6.90910 
6.14753 
8.23645 

I.638174 
1.638139 
I.63S161 
1.638099 
I.638114 

126.931 
126.926 
126.929 
126.920 
126.922 

Average 1.638137          126.925 

At  the  time  when  these  experiments  were  performed,  Richards  and  Wells  had 
not  completed  their  research  upon  the  atomic  weights  of  sodium  and  chlorine, 
so  that,  although  these  investigators  had  found  that  Stas's  determination  of  the 
atomic  weight  of  chlorine  had  yielded  too  low  results,  the  magnitude  of  his  error 
was  still  uncertain.  Using  a  value  for  chlorine  intermediate  between  Stas's  and 
Richards  and  Wells's  final  result,  the  above  five  experiments  in  Series  IV 
appeared  to  agree  almost  exactly  with  the  results  of  the  first  three  series  of 
experiments,  and  accordingly  all  four  series  were  pubHshed.  Not  long  afterwards 
Richards  and  Wells's  research  was  completed.  Using  their  final  result  for 
the  atomic  weight  of  chlorine,  35.457  (Ag  =  107.880),  as  has  been  done  in  this 
paper.  Series  IV  gives  a  result  o.oi  unit  higher  than  the  average  of  Series  II  and 
III.  The  desirabiUty  of  further  investigation  of  the  atomic  weight  of  iodine 
was  evident  and  new  experiments  were  accordingly  begun. 


A  REVISION  OF   THE   ATOMIC  WEIGHT  OF  IODINE.  105 

RATIO    OF   SILVER   IODIDE   TO   SILVER   BROMIDE. 

In  the  first  place  eight  experiments  were  made  in  which  the  iodide  was  heated 
in  a  current  of  air  and  bromine.  The  air  was  purified  by  being  conducted  suc- 
cessively over  beads  moistened  with  silver  nitrate  solution,  sodium  carbonate, 
and  finally  concentrated  sulphuric  acid  which  had  been  heated  to  its  boiling 
point  with  a  small  quantity  of  recrystallized  potassium  dichromate  to  eliminate 
volatile  and  oxidizable  impurities.  Four  different  samples  of  bromine  were  em- 
ployed, each  one  of  which  had  been  thrice  distilled  from  a  solution  of  a  bromide, 
in  each  distillation  the  bromide  having  been  made  from  a  portion  of  the  product 
of  the  previous  distillation.  Chlorine  must  have  been  completely  separated  in 
this  way.  The  presence  of  iodine  in  the  bromine  was  of  less  importance.  Nev- 
ertheless each  sample,  while  in  the  form  of  hydrobromic  acid,  was  freed  from 
iodine  by  boiling  with  several  small  portions  of  potassic  permanganate. 

The  apparatus  for  purifying  the  air  and  saturating  the  air  with  bromine  was 
so  constructed  that  the  gases  came  in  contact  only  with  glass.  The  quartz  cru- 
cibles were  always  contained  in  large  porcelain  crucibles  during  the  heating  in 
bromine,  as  well  as  in  the  initial  fusion  of  the  silver  iodide  with  iodine.  Vola- 
tiUzation  of  the  silver  halides  was  prevented  by  heating  very  gently  at  first,  at  a 
temperature  insufi&cient  to  fuse  the  mixture;  then,  when  the  greater  part  of  the 
iodine  had  been  replaced,  till  the  silver  bromide  barely  fused.  Moreover,  the  cru- 
cible was  very  deep  and  the  current  of  gases  very  slow,  so  that  any  volatilized 
silver  halides  had  opportunity  to  condense  upon  the  cool  walls  of  the  crucible. 

That  no  loss  from  volatilization  actually  took  place  was  certain  for  two  reasons. 
In  the  first  place,  the  weight  of  the  bromide  in  most  cases  became  constant  within 
a  few  hundredths  of  a  milligram  after  once  being  heated,  although  subsequently 
the  salt  was  maintained  at  a  temperature  slightly  above  its  melting  point  for  at 
least  an  hour.  In  the  second  place,  the  perforated  cover  and  hard-glass  delivery- 
tube  for  the  bromine,  when  rinsed  with  ammonia  and  the  solution  treated  with 
hydrochloric  acid  in  excess,  gave  no  visible  trace  of  opalescence.  Before  the  salt 
was  allowed  to  solidify  it  was  maintained  above  its  fusing  point  in  air  for  a  few 
minutes  to  eliminate  dissolved  bromine. 

The  silver  bromide  resulting  from  each  analysis  was  converted  into  silver  chlo- 
ride by  heating  in  a  current  of  pure  dry  chlorine  as  described  on  page  102.  Here 
also  constant  weight  within  a  very  few  hundredths  of  a  milligram  was  easily 
obtained,  and  as  before  the  perforated  cover  and  inlet  tube  were  free  from  weigh- 
able  amounts  of  chloride.  It  has  already  been  shown  that  silver  chloride  and 
silver  iodide,  when  fused  in  an  atmosphere  of  the  corresponding  halogens  and 
then  in  air,  retain  none  of  the  halogen.  The  quartz  crucibles  remained  almost 
absolutely  constant  in  weight  during  these  experiments.  From  the  ratio  be- 
tween the  original  silver  iodide  and  the  silver  chloride  the  atomic  weight  of  iodine 
was  calculated.  All  completed  analyses  are  recorded  in  the  tables.  Several 
were  lost,  owing  to  breaking  of  the  crucibles.  The  vacuum  correction  for  silver 
bromide  is  assumed  to  be  4-0.000041. 


io6 


RESEARCHES   UPON  ATOMIC  WEIGHTS. 


RESULTS. 

The  Atomic  Weight  OF  Iodine.       Series  V.    AgI:AgBr, 


No.  of 
analysis. 

Sample 

of 
bromine. 

Weight  of 
silver  iodide 
in  vacuum. 

Weight  of 

silver  bromide 

in  vacuum. 

Ratio. 
Agl  :  AgBr. 

Atomic  weight 
of  iodine. 

Ag  =  107.880. 
Br=    79.916. 

gm. 

gm. 

25 

A 

13.65450 

10.92087 

1. 250313 

126.924 

26 

A 

17-35521 

13-88056 

1.250325 

126.926 

27 

B 

9.70096 

7.75892 

1.250298 

126.921 

28 

B 

IO.27101 

8.21480 

1.250305 

126.922 

29 

B 

9.85684 

7.88347 

1. 250317 

126.925 

30 

C 

8.62867 

6.90102 

1-250347 

126.930 

31 

D 

11.92400 

9-53699 

1.250290 

126.919 

32 

D 

7-56930 

6.05386 

1.250326 

126.926 

Avera 

Lge    .... 

.    .1.250315 

126.924 

The  Atomic  Weight  of  Iodine. 

Series  VI.    AgI:AgCl. 

No.  of 
analysis. 

Weight  of 
silver  iodide 
in  vacuum. 

Weight  of 

silver  chloride 

in  vacuum. 

Ratio 
Agl :  AgCl. 

Atomic  weight 
CI  =  35-4S7- 

33 

34 
35 
36 
37 

gm. 
13.65450 
17-35521 
IO.27101 
8.62867 
11.92400 

gm. 
8.33536 
10.59453 
6.27004 
5.26732 
7.27923 

I.638142 
1. 638129 
I.638109 
I.638152 
1.63808s 

126.926 
126.925 
126.922 
126.928 
126.918 

Average 1.638123             126.924 

These  two  series  of  experiments  confirm  with  considerable  exactness  the 
higher  value  indicated  by  Series  IV,  and  lead  to  the  suspicion  that  owing  to  cer- 
tain pecuHarities  of  silver  iodide  which  have  already  been  discussed,  the  values 
furnished  by  the  earher  syntheses  of  silver  iodide  are  too  low.  Further  evidence 
was  therefore  sought  in  new  experiments  which  had  as  their  object  the  deter- 
mination of  the  ratio  of  iodine  to  both  silver  and  silver  iodide.  Agreement  be- 
tween the  sum  of  the  weights  of  the  iodine  and  the  silver  and  the  weight  of  the 
silver  iodide  would  furnish  important  evidence  as  to  the  accuracy  of  the  methods 
involved. 


A  REVISION  OF   THE  ATOMIC  WEIGHT   OF   IODINE.  I07 

RATIOS    BETWEEN    IODINE,    SILVER,   AND   SILVER 

IODIDE. 

For  this  purpose  iodine  was  purified  as  before  by  thrice  converting  it  into 
hydriodic  acid  with  hydrogen  sulphide,  and  then  heating  the  hydriodic  acid 
with  a  sHght  excess  of  potassium  permanganate  which  had  been  freed  from 
chlorine  by  crystallization.  The  final  product  of  iodine  was  distilled  once  with 
steam,  freed  from  water  by  suction  upon  a  porcelain  Gooch  crucible,  and  dried 
as  far  as  possible  in  a  desiccator  over  concentrated  sulphuric  acid.  It  was  then 
subHmed  once  from  porcelain  boats  in  a  current  of  pure  dry  air  in  a  hard-glass 
tube,  and  then  a  second  time  from  the  hard-glass  tube  into  the  weighing-tube, 
which  was  constructed  as  described  on  page  99  of  this  paper.  This  weighing- 
tube  remained  constant  in  weight  within  a  few  hundredths  of  a  milligram  in  each 
experiment,  and  lost  in  weight  in  all  only  o.i  mg.  in  the  eight  final  analyses. 

Next,  the  iodine  was  dissolved  in  sulphurous  acid,  and  was  then  precipitated 
by  adding  a  solution  of  a  slight  excess  of  pure  silver  to  the  solution  of  hydriodic 
acid  in  a  precipitating  flask.  The  silver  iodide  clotted  together  very  rapidly  in 
the  presence  of  the  nitric  acid  and  excess  of  silver,  and  offered  none  of  the  diffi- 
culties met  when  the  precipitation  is  carried  out  with  an  excess  of  iodide, 
for  it  may  be  washed  almost  indefinitely  with  pure  cold  water  without  showing 
the  least  tendency  to  pass  into  colloidal  solution.  The  silver  iodide  was  collected 
upon  a  Gooch  crucible,  dried,  and  weighed.  The  loss  on  fusion  was  determined, 
and  the  asbestos  shreds  in  the  filtrate  were  collected.  Twelve  determinations 
carried  out  in  this  way  with  solutions  varying  from  tenth  to  tliirtieth  normal  gave 
results  from  126.87  to  126.92.  The  variations  were  finally  traced  to  the  carrying 
down  of  silver  nitrate  by  the  silver  iodide;  for  the  more  concentrated  the  solu- 
tions during  the  precipitation  and  the  greater  the  excess  of  silver  employed,  the 
lower  was  the  observed  atomic  weight  of  iodine.  The  occlusion  of  silver  nitrate 
by  silver  halides  is  well  known  in  the  cases  of  silver  bromide  and  silver  chloride, 
but  in  these  cases  it  is  possible  to  wash  out  the  occluded  salts  by  oft-repeated 
treatment  with  water.  The  difficulty  in  removing  the  silver  nitrate  in  the  case 
of  silver  iodide  is  doubtless  due  to  the  lesser  solubility  of  this  halide. 

Similar  observations  have  been  made  also  by  Kothner  and  Aeuer,^  who  found 
that  with  fifth  normal  solutions  the  carrying  down  of  silver  nitrate  is  very  con- 
siderable, and  that  an  excess  of  iodide  converts  this  occluded  silver  nitrate  into 
silver  iodide  only  very  slowly  if  at  all.  They  found,  however,  that,  if  the  pre- 
cipitated silver  iodide  is  washed  with  ammonia,  the  greater  part  of  the  occluded 
matter  may  be  eliminated,  owing  possibly  to  the  slight  solubility  of  silver 
iodide  in  ammonia.  On  account  of  this  newly  discovered  tendency  of  silver 
iodide,  it  seemed  probable  that  the  results  of  the  synthesis  of  silver  iodide  from 
known  weights  of  silver  in  the  early  part  of  this  research  were  somewhat  too  low; 
for  although  the  precipitations  took  place  in  ammoniacal  solution  and  an  excess 

1  Liebig's  Ann.,  337, 123  (1904). 


io8 


RESEARCHES  UPON  ATOMIC  WEIGHTS. 


of  iodine  was  employed,  the  fact  that  the  solutions  were  always  very  nearly 
as  strong  as  tenth  normal  and  that  the  ammonic  iodide  was  poured  into  the 
silver  nitrate  made  it  probable  that  occlusion  had  taken  place  at  least  to  some 
extent  even  here.  It  is  noteworthy  that  three  of  these  syntheses  yielded  results 
as  high  as  126.926.  Similar  conclusions  are  to  be  drawn  concerning  the  results 
of  the  titration  of  silver  and  iodine,  although  in  this  case  precipitation  took  place 
in  the  reverse  fashion,  i.  e.,  by  adding  the  silver  nitrate  to  the  iodide. 

Accordingly  experiments  were  carried  out  for  the  redetermination  of  the 
ratios  of  silver  to  silver  iodide  and  to  iodine,  as  well  as  the  new  one  of  iodine  to 
silver  iodide,  with  especial  precautions  to  avoid  the  error  of  occlusion  by  using 
very  dilute  solutions  and  no  excess  of  silver.  After  the  iodine  had  been  weighed, 
very  nearly  the  exact  amount  of  silver  to  combine  with  the  iodine  was  weighed 
out  and  dissolved  in  nitric  acid  with  the  usual  precautions  to  avoid  spattering. 
The  quantity  of  iodine  used  in  each  analysis  was  between  3  and  4  gm.,  and  the 
amount  of  silver  consequently  about  3  gm. 

The  solutions  of  both  silver  and  iodine  were  diluted  to  a  volmne  of  i  liter  each, 
so  that  the  solutions  were  about  thirtieth  normal,  and  then  the  silver  nitrate 
was  added  very  slowly  to  the  solution  of  hydriodic  acid  with  constant  stirring. 
The  flask  in  which  precipitation  was  carried  out  was  then  shaken  for  some  time, 
and  allowed  to  stand  until  the  supernatant  liquid  was  clear.  This  liquid  was 
tested  for  an  excess  of  iodine  or  silver  in  a  nephelometer,  and  if  a  deficiency  of 
either  was  found,  it  was  made  up  and  the  solution  again  shaken,  until  the  point 
was  reached  where  the  extremely  faint  opalescence  produced  by  both  hydriodic 
acid  and  silver  nitrate  was  equal  in  both  nephelometer  tubes.  The  excess  or 
deficiency  of  silver  was  in  no  case  over  o.i  mg.,  and  in  most  cases  much  less. 

The  Atomic  Weight  of  Iodine.       Series  VII.    Ag:  I. 


No.  of 

Sample  of 

Weight  of  iodine 

Weight  of  sil- 

Ratio 

Atomic  weight 

analysis. 

silver. 

in  vacuum. 

ver  m  vacuum. 

Ag:I. 

of  iodine. 

gm. 

gm. 

38 

A 

3.29307 

2.79897 

0.849958 

126.924 

39 

B 

3.70131 

3-14584 

0.849926 

126.929 

40 

B 

3-75640 

3-19258 

0.849904 

126.932 

41 

B 

3-24953 

2.76186 

0.849926 

126.929 

42 

A 

4-12539 

3-50639 

0.849954 

126.925 

43 

C 

3-53165 

3.00165 

0.849928 

126.929 

44 

C 

2.99834 

2.54842 

0.849944 

126.926 

45 

C 

2.00014 

I.69991 

0.849896 

126.933 

Average 

.      0.849930 

126.928 

Several  samples  of  iodine  were  employed  in  these  analyses,  each  one  of  which 
had  been  three  times  distilled  from  an  iodide  as  previously  described.  Three 
different  specimens  of  silver,  purified  by  different  methods,  were  used.  Sample 
A  was  a  portion  of  the  material  used  in  the  early  part  of  this  investigation. 
Sample  B  was  prepared  from  silver  nitrate  which  had  been  recrystallized  seven 
times  from  nitric  acid,  five  times  from  water,  and  finally  precipitated  by  am- 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF   IODINE. 


109 


monium  formate.  Sample  C  was  precipitated  as  silver  chloride  once,  electro- 
lyzed  once,  and  finally  precipitated  with  formic  acid.  All  three  samples  were 
fused  in  a  current  of  hydrogen  on  a  lime  boat. 

In  order  to  determine  whether  an  even  more  gradual  addition  of  silver  nitrate 
would  influence  the  extent  of  the  occlusion,  in  Analysis  42  the  silver  was  intro- 
duced by  means  of  a  large  funnel  provided  with  a  fine  spout. 

In  one  of  the  foregoing  analyses  the  attempt  was  made  to  determine  the  ratio 
between  iodine  and  silver  iodide  by  adding  an  excess  of  silver  nitrate  and  weigh- 
ing the  precipitate.  Although  only  5  mg.  excess  of  silver  nitrate  were  added, 
while  the  total  volimae  of  the  liquid  was  over  2  liters,  the  precipitate  was  i  mg. 
heavier  than  it  should  have  been  as  calculated  from  the  result  of  the  titration, 
showing  that  the  occlusion  of  silver  nitrate  by  the  silver  iodide  may  take  place 
even  after  precipitation.  In  order  to  avoid  this  difficulty,  in  succeeding  anal- 
yses, the  silver  iodide  was  collected  without  the  addition  of  silver  nitrate,  after 
washing  with  pure  water;  then  the  dissolved  silver  iodide  was  determined  in  the 
filtrate  and  wash-water,  by  comparing  the  precipitates  produced  by  the  addi- 
tion of  silver  nitrate  with  those  formed  in  standard  solutions  of  hydriodic  acid. 
As  a  matter  of  fact,  the  filtrates  were  always  found  to  be  practically  free  from 
silver  iodide,  while  the  wash-water  contained  from  0.2  to  0.4  of  a  mg.  per  liter. 
This  difference  in  solubility  of  the  silver  iodide  in  the  filtrate  and  wash-waters 
is  probably  due  to  colloidal  solubility  of  this  salt  in  pure  water.  The  loss  on 
fusion  of  the  silver  iodide  and  the  asbestos  shreds  were  determined  as  usual.  The 
almost  absolute  lack  of  opalescence  in  the  filtrate  when  treated  with  silver 
nitrate  or  hydriodic  acid  is  a  good  proof  of  the  complete  absence  of  chlorine 
and  bromine,  since  both  silver  bromide  and  silver  chloride  are  considerably 
more  soluble  than  the  iodide. 


The  Atomic  Weight  OF  Iodine.     Series  VIII.    I:AgI. 


No.  of 
analysis. 

Weight  of 
iodine  in 
vacuum. 

Corrected 

weight  of 

silver  iodide 

in  vacuum. 

Ratio 
I:AgI. 

Atomic 

weight  of 

iodine. 

Ag  =107.880. 

46 
47 
48 
49 
50 

gm. 
3-75640 
3-24953 
4.12539 
3-53165 
2.99834 

gm. 
6.94911 
6.0113s 
7.63201 
6.53348 
5.54680 

0.540558 
0.540566 
0.540538 
0-540547 
0.540553 

126.927 
126.931 
126.916 
126.921 
126.924 

Average 0.540552         126.924 

Finally,  in  order  to  determine  the  ratio  of  silver  to  silver  iodide,  the  fil- 
trate and  wash-waters  were  evaporated  to  very  small  bulk,  until  the  greater 
part  of  the  nitric  acid  had  been  expelled,  and  then  the  amount  of  silver  in  the 
residue  was  determined  nephelometrically,  after  dilution  to  25  c.c,  by  adding 
an  excess  of  hydriodic  acid  and  comparing  with  standard  solutions  of  silver. 


no 


RESEARCHES  UPON  ATOMIC  WEIGHTS. 


This  quantity,  which  was  never  more  than  o.i  mg.,  was  subtracted  from  the 
original  weight  of  silver,  and  no  correction  was  applied  to  the  weight  of  silver 
iodide  for  the  amount  dissolved  in  the  wash-water. 


The  / 

LTOMic  Weight 

OF  Iodine.    Series  IX.    Ag:AgI. 

No.  of 
analysis. 

Sample  of 
silver. 

Corrected 

weight  of  silver 

in  vacuum. 

Corrected 

weight  of  silver 

iodide  in  vacuum. 

Ratio 
Ag:AgI. 

Atomic  weight 

of  iodine. 
Ag  =  107.880. 

SI 

52 

S3 
54 

B 
B 
C 
C 

gm. 
3.19249 

2.7617s 
3.00189 

2.34833 

gm. 
6.9487s 
6.01 108 
6.53396 
5.54657 

0.459434 
0.459443 
0.459429 
0.459443 

126.931 
126.926 
126.933 
126.926 

Average    ....    0.459437               126.929 

DISCUSSION   OF   RESULTS. 

In  the  following  table  are  summarized  the  results  from  the  different  series: 


Series. 

Material. 

Result. 

Series. 

Material. 

Result. 

I 

II 

III 

IV 

V 

Ag:AgI 

Ag:AgI 

Ag:I 

Agl:  AgCl  direct 

Agl:  AgBr 

126.899 
126.913 
126.918 
126.925 
126.924 

VI 

VII 

VIII 

IX 

Agl:  AgCl  indirect 
Ag:I 
I:  Agl 
Ag:AgI 

126.924 
126.928 
126.924 
126.929 

Average,  omitting  Series  I.  II.  and  I] 

[I   .  .  . 

126.926 

In  computing  the  atomic  weight  of  iodine  from  these  data,  Series  I,  II,  and 
III  should  obviously  be  neglected,  since  the  experiments  in  these  series  are 
superseded  by  the  more  accurate  ones  of  Series  VII  and  IX. 

In  Series  IV,  V,  and  VI,  the  three  chief  possible  errors  have  been  considered 
and  shown  to  have  no  effect.  Occluded  silver  nitrate  in  the  silver  iodide  was 
eliminated  by  fusion  in  iodine;  and  it  was  proved  both  that  no  volatilization  of 
the  halides  actually  took  place  and  that  no  halogen  was  retained  by  the  solidified 
salt.  In  Series  VII,  VEII,  and  IX,  the  chief  possible  error,  that  of  occlusion  of 
silver  nitrate  by  silver  iodide,  was  avoided  by  sufficiently  diluting  the  solutions 
before  precipitation.  That  the  dilution  was  sufficient  was  evident  from  the 
agreement  of  the  results  of  the  experiments  with  the  larger  and  smaller  quanti- 
ties of  material,  the  total  volume  of  the  solutions  being  the  same  in  all  cases. 
Furthermore,  the  weight  of  the  silver  iodide  obtained  in  the  analyses  recorded  in 
Series  VIII  was  in  every  case  very  nearly  equal  to  the  sum  of  the  weights  of  silver 
and  iodine  employed  (Series  VII),  which  could  not  have  been  the  case  if  silver 


A  REVISION  OP  THE  ATOMIC  WEIGHT  OF  IODINE. 


Ill 


nitrate  had  been  retained  by  the  iodide.  Additional  evidence  upon  this  point  is 
afforded  by  the  percentages  of  iodine  and  silver  in  silver  iodide  as  given  in  Series 
VIII  and  IX,  for  the  sum  of  the  two  percentages  is  0.999989,  the  discrepancy 
being  only  a  trifle  greater  than  o.ooi  per  cent.  The  close  agreement  of  the  aver- 
ages of  the  last  six  series  makes  it  certain  that  no  constant  error  of  magnitude 
still  remains  undetected  in  any  one  of  the  methods.  One  is  forced,  then,  to  con- 
clude that  the  average  of  the  last  six  series,  126.926,  represents  the  atomic  weight 
of  iodine  within  a  very  few  thousandths  of  a  unit. 

RATIO   OF   SILVER   BROMIDE   TO   SILVER   CHLORIDE. 

It  is  interesting,  also,  to  compare  the  weights  of  silver  bromide  and  silver 
chloride  produced  in  the  same  analysis.  When  this  is  done  the  ratio  of  silver 
bromide  to  silver  chloride  and  the  atomic  weight  of  bromine  computed  with  the 
assumed  atomic  weight  of  chlorine,  35.457,  are  found  to  be  identical  with  the 
corresponding  values  subsequently  determined  by  Baxter.^ 

Series  X.    AgBr:  AgCl. 


Sample 

Weight  of 

Weight  of 

Atomic 

of 

silver  bromide 

silver  chloride 

AgBr :  AgCl. 

weight  of 

bromine. 

m  vacuum. 

m  vacuum. 

bromine. 

gm. 

gm. 

A 

10.92087 

8.33536 

I.310186 

79.918 

A 

13  -88056 

10.59453 

I.310163 

79.915 

B 

8.21480 

6.27004 

I.310167 

79.915 

B 

7.87883 

6.01350 

I.310190 

79.919 

C 

6.90102 

5.26732 

1.310158 

79.914 

D 

9-53699 

7.27923 

I.310164 

79.915 

A^ 

yeraere  .        ^    . 

.    I.310171 

79.916 

See  page  61. 


112  RESEARCHES  UPON  ATOMIC  "WEIGHTS. 


HISTORICAL   DISCUSSION. 


In  discussing  the  bearing  of  this  investigation  upon  the  results  of  earlier  work 
by  other  chemists,  the  experiments  of  Millon  upon  silver  and  potassium  iodates  ^ 
and  of  Berzehus  ^  and  Dumas,^  who  converted  silver  iodide  into  silver  chloride, 
may  be  disregarded,  since  at  the  time  the  analyses  were  made,  quantitative 
analysis  was  in  its  infancy.  Marignac's  ^  value  for  the  atomic  weight  of  iodine, 
126.79,  obtained  from  titration  of  weighed  amounts  of  silver  with  potassic 
iodide,  and  from  syntheses  of  silver  iodide  from  weighed  quantities  of  silver, 
may  be  accounted  for  by  the  supposition  that  the  iodine  used  in  the  experiments 
was  not  pure.  To  explain  Stas's  ^  low  value,  126.79,  is  a  difficult  matter.  His 
iodine  was  purified  by  two  different  methods,  i.  e.,  by  once  precipitating  or  dis- 
tilling the  iodine  from  a  strong  solution  of  potassic  iodide,  and  by  precipitation 
of  nitrogen  iodide.  A  third  sample  was  purified  by  both  methods.  Since  the 
material  purified  by  each  of  the  two  methods  gave  identical  results  with  that 
purified  by  both  methods,  it  is  inconceivable  that  either  method  of  purification 
should  not  have  been  effective.  Impurity  in  the  silver  or  loss  of  silver  iodide  are 
improbable  causes  of  the  discrepancy,  for  the  weight  of  the  silver  iodide  produced 
was  equal  to  the  simi  of  the  weights  of  the  silver  and  iodine  employed.  Rich- 
ards and  Wells  have  recently  shown,  however,  that  Stas  was  not  infalHble,  and 
in  fact  was  capable  of  making  serious  mistakes,  such  that  his  value  for  the  atomic 
weight  of  sodium  was  0.2  per  cent  too  high,  and  that  for  the  atomic  weight  of 
chlorine  0.03  per  cent  too  low,  so  that  it  is  not  at  all  surprising  to  find  that  here 
also  his  work  was  faulty  in  some  undiscovered  particular. 

Ladenburg's  result,^  when  calculated  from  the  true  value  of  the  atomic  weight 
of  chlorine, becomes  126.988,  which  agrees  very  closely  with  the  value  deduced  in 
this  paper.  His  determinations  were  affected  by  several  smaU  errors,  so  that  the 
close  agreement  is  somewhat  the  result  of  chance.  In  the  first  place,  porcelain  cru- 
cibles, as  has  been  pointed  out  before,  gain  in  weight  when  used  for  the  conversion 
of  silver  iodide  into  silver  chloride,  so  that  the  weight  of  the  silver  chloride  is 
somewhat  uncertain.  Furthermore  Ladenburg  did  not  fuse  the  silver  iodide 
before  weighing  it.  Although  Stas  states  that  silver  iodide  may  be  completely 
dried  without  fusion,  his  experiments  show  a  loss  in  weight  on  fusion  of  0.002 
per  cent,  while  the  average  loss  on  fusion  as  given  on  page  97  is  about  0.004  per 
cent.  Moreover,  Ladenburg's  method  of  purifying  silver  iodide,  by  washing  the 
precipitated  salt  with  ammonia,  could  hardly  be  expected  to  remove  last  traces 
of  silver  chloride  and  silver  cyanide  "included"  by  the  precipitate.  These 
errors  are  all  so  small  that  they  would  not  affect  the  second  place  of  decimals. 

Shortly  after  the  publication  of  the  first  portion  of  this  research  there  ap- 
peared the  complete  paper  of  Kothner  and  Aeuer  upon  the  same  subject.''  These 

^  Ann.  de  Chem.  et  de  Pkys.  (3)  9,  400  (1843).       ^  Ibid.,  (2)  40,  430  (1829). 
^  Ann.  Chem.  Pharm.,  113,  28  (i860).  *  Berzelius'  Lehrbuch,  sth  ed.,  3,  1196. 

*  (Eiivres  Completes,  i,  548.  '  Ber.  d.  d.  Chem.  Gesell.,  35,  2275  (1902). 

^  Liebig's  Ann.,  337,  123  (1904). 


A  REVISION  OF   THE   ATOMIC  WEIGHT  OF   IODINE.  II3 

chemists,  determined  the  ratio  Agl:  AgCl,  in  eight  closely  agreeing  analyses. 
One  synthesis  was  made  by  precipitating  a  weighed  amount  of  silver  with 
hydriodic  acid,  and  one  by  heating  a  weighed  amount  of  silver  in  iodine.  A 
second  paper*  by  the  same  authors  contains  a  recalculation  of  their  results  with 
a  different  value  for  chlorine,  and  a  critical  discussion  of  their  own,  Ladenburg's, 
and  my  work. 

In  this  second  paper  two  main  criticisms  of  my  investigation  are  made.  One 
of  these  concerns  the  vahdity  of  the  conclusion  that  ordinary  iodine  does  not 
contain  an  undiscovered  halogen  element.  This  criticism  is  founded  upon 
a  mistaken  understanding,  however.  My  experiments  were  directed  to  prove 
the  existence  or  non-existence  of  an  element  of  Mg/fer  atomic  weight  thah  iodine, 
which,  if  its  properties  were  those  to  be  expected  from  the  properties  of  the 
other  members  of  the  halogen  family,  would  have  been  set  free  from  solutions 
of  its  compounds  by  iodine,  and  hence  would  have  accumulated  in  the  first 
fraction  of  the  fractional  separation  (see  page  93).  The  existence  of  an  im- 
known  halogen  of  lower  atomic  weight  than  iodine  was  not  considered. 

Although  Kothner  assumes  the  possibiHty  that  such  an  element  exists,  it  is 
obviously  improbable  that  any  halogen  of  lower  atomic  weight  could  have  re- 
mained in  the  purified  samples  of  iodine  which  were  employed  in  my  experi- 
ments; since  those  specimens  which  received  even  the  least  purification  were 
thrice  distilled  from  an  iodide,  the  iodide  having  been  made  in  each  distilla- 
tion from  nearly  half  the  iodine  from  the  previous  distillation. 

In  the  second  place,  it  is  maintained  that  volatilization  of  silver  halides  takes 
place  when  silver  iodide  is  heated  in  a  current  of  chlorine.  This  is  undoubtedly 
true,  unless  precautions  are  taken  to  prevent  the  volatilization.  Silver  iodide  is 
much  more  volatile  at  its  fusing  temperature  than  silver  chloride;  and  if  the 
original  silver  iodide  is  fused  before  the  current  of  chlorine  or  bromide  is  begun, 
a  loss  may  take  place  by  volatilization  or  possibly  by  spattering.  In  my  own 
work,  however,  fusion  of  the  salt  was  always  avoided  until  the  greater  part  of 
the  change  had  taken  place.  Proof  that  no  volatilization  of  silver  halides  actually 
occurred  in  my  experiments  has  already  been  given  (page  105).  It  may  be  added 
that  the  exact  agreement  of  the  results  of  Series  V  and  VI  is  additional  evi- 
dence in  the  same  direction;  for  any  loss  of  silver  bromide,  which  is  the  most 
volatile  of  the  three  halides,  would  have  tended  to  raise  the  results  of  Series  V 
above  those  of  Series  VI. 

Kothner's  average  result  in  the  series  Agl:  AgCl,  when  recalculated  upon  the 
basis  of  chlorine  35.457,  becomes  126.915.  The  slight  difference  between  this 
value  and  the  final  result  of  the  research  described  in  this  paper  may  be  explained 
in  several  possible  ways.  A  perusal  of  Kothner's  paper  does  not  make  clear 
whether  or  not  fusion  of  the  silver  iodide  took  place  at  the  beginning  of  his  ex- 
periments, so  that  it  is  uncertain  whether  the  gain  in  weight  of  the  coil  of  glass 

*  Liebig's  Ann.,  337,  362  (1904). 


114  RESEARCHES    UPON    ATOMIC    WEIGHTS. 

tubing,  which  was  attached  to  the  reaction  tube,  was  due  to  volatilized  silver 
halides,  or  to  attacking  of  the  glass  by  the  hot  halogens.  At  any  rate,  it  is  hard 
to  believe  that  1.5  meter  of  glass  tubing  subjected  to  the  long-continued  action 
of  mixed  chlorine  and  iodine  at  150°  should  not  have  altered  its  weight  some- 
what. Furthermore,  Kothner  himself  showed  that  the  tube  in  which  the  reac- 
tion took  place  was  slightly  attacked  by  the  fused  silver  chloride,  but  the  nature 
of  the  correction  for  this  attacking  of  the  glass  is  uncertain.  Finally,  although 
the  precaution  was  taken  of  fusing  the  silver  iodide  before  the  initial  weighing, 
so  that  the  salt  must  have  been  free  from  moisture,  it  was  not  fused  in  an  atmos- 
phere of  iodine.  Hence  it  is  possible  that  the  iodide  still  contained  traces  of 
occluded  silver  nitrate  (or  metallic  silver).  This  deficiency  in  iodine,  as  well  as 
the  possible  gain  in  weight  of  the  apparatus  during  the  experiment,  would  have 
lowered  the  atomic  weight  of  iodine. 

As  far  as  Kothner's  sjmtheses  of  silver  iodide  from  weighed  amount  of  silver 
are  concerned,  it  need  only  be  said  that,  even  assuming  that  occlusion  was 
avoided  in  the  synthesis  in  the  wet  way,  and  that  in  the  synthesis  in  the  dry 
way  a  single  precipitation  of  the  iodine  from  solution  in  an  iodide  had  completely 
removed  such  impurities  as  chlorine  and  bromine,  which  would  have  accumulated 
in  the  silver  iodide  during  the  experiment,  the  fact  that  Richards  and  Wells 
have  shown  that  silver  fused  in  the  air  or  with  borax  and  saltpetre,  according  to 
Stas,  must  contain  oxygen,  makes  it  certain  that  the  results  of  Kothner's  syn- 
theses, 126.919  and  126.904,  are  too  low. 

It  is  interesting  to  note,  however,  that  Ladenburg's,  Scott's,  and  Kothner 
and  Aeuer's  work  all  afford  confirmation  that  the  atomic  weight  of  iodine  is 
undoubtedly  much  higher  than  was  formerly  supposed  from  Stas's  syntheses  of 
silver  iodide. 

SUMMARY. 

The  residts  of  the  research  are,  then,  as  follows: 

1.  The  atomic  weight  of  iodine  is  found  to  be  126.926(0  =  16.000  and  Ag  = 
107.880).   If  silver  is  taken  at  107.870,  iodine  becomes  126.914. 

2.  The  existence  of  an  element  of  the  halogen  family  of  higher  atomic  weight 
than  iodine  is  shown  to  be  improbable. 

3.  The  specific  gravity  of  pure  fused  silver  iodide  is  found  to  be  5.674  at  25° 
referred  to  water  at  4°. 

4.  The  observation  by  Kothner  and  Aeuer  that  under  certain  conditions  sil- 
ver iodide  occludes  silver  nitrate,  and  that  this  occluded  salt  can  not  be  removed 
by  washing  with  water,  is  confirmed. 

5.  The  value  of  Richards  and  Wells  for  the  atomic  weight  of  chlorine 
35.457  (Ag  =  107.880)  and  Baxter's  for  the  atomic  weight  of  bromine  79.916 
are  substantiated. 


VIII. 

A    REVISION    OF   THE   ATOMIC   WEIGHTS   OF 
IODINE   AND   SILVER. 

THE   ANALYSIS    OF  IODINE  PENTOXIDE. 


By  Gregory  Paul  Baxter  and  George  Stephen  Tilley. 


Journal  of  the  American  Chemical  Society,  31,  201  (1909). 
Zeitschrift  fiir  anorganische  Chemie,  61,  293  (1909). 
Chemical  News,  100,  259,  261,  274,  286,  310  (1909). 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A  REVISION  OF  THE  ATOMIC  WEIGHTS  OF  IODINE 

AND  SILVER. 

THE   ANALYSIS   OF  IODINE   PENTOXIDE. 


INTRODUCTION. 


For  some  time  it  has  been  apparent  that  Stas's  researches  upon  the  atomic 
weight  of  silver,  upon  which  the  value  now  in  use  depends,  are  somewhat  at 
fault.  The  need  for  a  redetermination  of  the  ratio  of  this  constant  to  the  atomic 
weight  of  oxygen  is  especially  pressing,  since  silver  has  been  very  frequently 
used  as  the  basis  for  exact  work  upon  atomic  weights,  both  directly,  and  also 
indirectly  through  its  relation  to  the  atomic  weights  of  the  halogens  in  the  analy- 
sis of  metalUc  halides. 

The  problem  is  made  difficult,  however,  by  the  fact  that  the  only  known  definite 
compounds  of  silver  with  oxygen  are  difficult  if  not  impossible  to  prepare  in  a 
pure  state,  besides  containing  so  small  a  percentage  of  oxygen  that  the  ex- 
perimental error  would  be  greatly  magnified  in  the  calculation.  All  other  meth- 
ods involve  the  knowledge  of  the  exact  ratio  of  at  least  one  other  atomic  weight 
to  that  of  silver  or  oxygen,  the  accuracy  of  the  process  diminishing  with  the 
number  of  atoms  involved.  Among  the  various  classes  of  compounds  the 
analysis  of  which  may  afford  the  desired  information,  oxides  are  of  especial 
interest  because  the  knowledge  of  the  ratio  of  only  one  atomic  weight  to  that  of 
silver  is  involved.  The  difficulty  and  uncertainty  in  analyzing  metallic  oxides  ^ 
make  them  unsuited  for  the  purpose.  The  ratios  of  the  atomic  weights  of  silver 
and  the  halogens  are  known  with  greater  exactness  than  in  the  case  of  most 
elements,  hence  to  determine  the  ratio  of  any  one  of  the  halogens  to  that  of 
oxygen  will  serve  the  purpose  equally  well.  The  only  compound  of  a  halogen 
and  oxygen  of  considerable  stability  is  iodine  pentoxide.  This  substance  is, 
however,  quite  stable  through  a  fairly  wide  range  of  conditions,  and  its  analysis 
offered  enticing  possibilities.  To  be  sure,  the  compound  contains  only  25  per 
cent  of  oxygen,  but  this  disadvantage  is  in  part  counterbalanced  by  the  fact 
that  the  ratio  of  the  atomic  weight  of  iodine  to  that  of  silver  is  greater  than  in 
the  case  of  most  elements. 

^  Richards  and  Baxter:  Proc.  Amer.  Acad.,  35,  61  (1899);  35,  253  (1900);  Zeii.  anorg. 
Ckem.,  22,  221;  23,  245. 

117 


Il8  RESEARCHES   UPON  ATOMIC  WEIGHTS. 

The  method  actually  employed  was  to  reduce  to  hydriodic  acid  weighed 

amounts  of  iodine  pentoxide,  and  to  determine  the  weight  of  silver  necessary 

zAg 
to  combine  with  the  iodine.    If  the  ratio  of  silver  to  iodine  pentoxide,  — -,  is 

multiplied  by  that  of  iodine  to  silver,  -— ,  the  per  cent  of  iodine  in  the  pent- 

Ag 

2I 
oxide  IS  obtained,  and  thence  the  ratio  — . 

5O 

PURIFICATION    OF   MATERIALS    FOR   THE    FIRST    SERIES    OF 

ANALYSES. 

IODIC  ACID. 

Iodine  pentoxide  may  be  readily  prepared  by  dehydration  of  iodic  acid  at  a 
comparatively  low  temperature.  Of  the  various  methods  for  preparing  the 
latter  substance,  by  far  the  most  satisfactory  for  the  present  purpose  is  the 
oxidation  of  iodine  with  fuming  nitric  acid.  The  volatile  by-products  of  the 
reaction,  oxides  of  nitrogen  and  water,  as  well  as  the  excess  of  nitric  acid,  may 
be  removed  by  heating,  and  the  acid  may  be  crystallized  from  aqueous  solution. 

Iodine  was  purified  for  most  of  the  preparations  by  the  methods  already 
described  by  one  of  us.^  These  consisted  usually  in  a  preliminary  distillation  of 
commercial  iodine  from  solution  in  commercial  aqueous  potassiimi  iodide. 
The  iodine  was  then  twice  reduced  to  hydriodic  acid  by  hydrogen  sulphide 
and  water,  and  the  hydriodic  acid  was  subsequently  oxidized  by  very  nearly 
the  theoretical  quantity  of  recrystallized  permanganate,  with  intermediate 
boiling  of  the  hydriodic  acid  solution  to  eliminate  cyanogen  as  hydrocyanic 
acid.2  In  this  way,  since  only  five-eighths  of  the  iodine  is  set  free  from  hydriodic 
acid  by  permanganate,  the  iodine  was  three  times  distilled  from  a  solution  of  an 
iodide,  the  iodide  being  of  greater  purity  in  each  succeeding  distillation.  It  has 
already  been  shown  that  three  such  distillations  are  sufficient  to  remove  com- 
pletely chlorine  and  bromine,^  so  that  there  can  be  no  question  of  the  freedom  of 
the  resulting  iodine  from  those  closely  related  elements.  The  final  product 
was  once  distilled  with  water  into  a  Jena  glass  flask  cooled  with  running  water, 
and  the  resulting  solid  was  washed  with  the  purest  water  upon  a  porcelain 
Gooch  crucible  containing  no  mat. 

Pure  fuming  nitric  acid  was  prepared  by  distilling  the  best  commercial 
product  several  times,  with  rejection  of  the  first  and  last  thirds,  until  the  distil- 
late, after  dilution,  gave  no  test  for  chloride  with  silver  nitrate  in  a  nephelo- 

^  Baxter:  Froc.  Amer.  Acad.,  40,  421  (1904);  Jour.  Amer.  Chem.  Soc,  26, 1579;  Zeit.  anorg. 
Chetn.,  43, 16.    (See  page  92.) 

*  Richards  and  Singer:  Amer.  Chem.  Jour.,  27,  205  (1902). 

3  Baxter:  Loc.  cit.;  aXso  Proc.  Amer.  Acad.,  41,  73  (1905);  Jour.  Amer.  Chem.  Soc,  27,  876; 
Zeit.  anorg.  Chem.,  46,  36. 


A  REVISION  OF  THE   ATOMIC  WEIGHTS   OF  IODINE   AND   SILVER.         IIQ 

meter.  A  quartz  condensing  tube  was  used  in  these  distillations,  and  the  final 
product  was  collected  in  a  quartz  vessel.  In  this  way  the  introduction  of  silica 
and  alkalies  from  glass  vessels  was  avoided.  Fused  quartz  vessels  have  already 
been  shown  to  be  essentially  insoluble  in  acid  solutions.^ 

The  reaction  between  iodine  and  fuming  nitric  acid  proceeds  slowly  at  ordi- 
nary temperatures,  with  the  formation  of  oxides  of  nitrogen  and  nitro-iodic  acid. 
Heat  hastens  the  reaction  and  also  breaks  up  the  nitro-iodic  acid  into  nitric  oxide, 
iodine,  and  iodic  acid.  If  the  nitric  acid  is  in  large  excess  the  iodine  is  converted 
into  iodic  acid  without  loss,  and  the  iodic  acid  remains  insoluble  in  the  residual 
concentrated  nitric  acid.  If  the  iodine  is  in  excess,  the  reaction  proceeds  until 
the  nitric  acid  is  so  dilute,  owing  to  the  production  of  water  in  the  reaction,  as  to 
be  without  fvirther  effect  upon  the  iodine,  although  the  acid  is  still  so  concen- 
trated that  the  iodic  acid  remains  essentially  insoluble.  Nitric  acid  of  this 
concentration  dissolves  considerable  quantities  of  iodine,  however. 

Since  the  largest  quartz  vessel  at  first  available  was  aiooc.c.  transparent  fused 
quartz  flask,  the  most  convenient  method  for  making  the  iodic  acid  was  found  to 
be  to  treat  a  large  quantity  of  iodine  in  the  flask  with  successive  portions  of 
fuming  nitric  acid.  After  the  introduction  of  the  iodine,  nitric  acid  was  dis- 
tilled directly  into  the  flask  through  the  quartz  condenser.  The  flask  was  then 
warmed  until  the  nitric  acid  was  spent.  The  spent  acid  was  removed  as  com- 
pletely as  possible  by  drainage,  and  was  replaced  by  fresh  acid,  and  the  process 
was  repeated  until  the  iodine  appeared  to  be  completely  oxidized.  Dissolved 
iodine  was  recovered  from  the  spent  acid  by  dilution.  After  the  removal  of  the 
last  portion  of  the  acid,  the  iodic  acid  was  dissolved  in  the  smallest  possible 
amount  of  the  purest  water  and  the  solution  was  evaporated  to  dryness  in  a 
dish  of  fused  quartz  in  order  to  expel  nitric  acid  and  unchanged  iodine,  for  the 
removal  of  nitric  acid  by  crystallization  is  very  slow.  During  this  evaporation 
the  dish  was  placed  upon  a  large  watch  glass  on  a  sand  bath,  and  was  surrounded 
by  a  large  bottomless  beaker.  The  dish  was  further  protected  by  being  cov- 
ered with  a  bottomless  flask,  through  the  neck  of  which  a  ciurrent  of  pure  dry 
air  was  introduced  in  order  to  hasten  evaporation.  The  air  was  freed  from 
organic  matter  by  passing  over  hot  copper  oxide  in  a  hard-glass  tube,  and  was 
purified  and  dried  by  means  of  a  solution  of  potassium  hydroxide  and  solid 
caustic  potash.    The  purifying  apparatus  was  constructed  wholly  of  glass. 

The  residue  from  the  evaporation  was  dissolved  in  the  purest  water  and  evap- 
orated in  the  quartz  dish  with  the  same  precautions  as  above  imtil  a  film  of 
solid  appeared  on  the  surface  of  the  liquid.  This  solid  is  not  iodic  acid,  but  is 
probably  a  compound  having  less  water,  since  it  was  necessary  to  induce  the 
crystallization  of  the  acid  itself  by  inoculation.  If  left  to  itself  the  acid  crystal- 
lizes very  slowly,  some  days  being  necessary  for  the  establishment  of  equilib- 

*  Mylius  andMeusser;  Zeit.  anorg.  Chem.,  44,  221  (1905).  Baxter  andHines:  Jour.  Atner. 
Chem.  Soc,  28, 1565  (1906);  Zeit.  anorg.  Chem.,  51,  205.    (See  page  36.) 


120  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

rium.  It  is  economical,  however,  to  wait  until  crystallization  has  ceased,  since 
the  solubility  of  the  acid  at  room  temperature  is  so  high,  a  saturated  solution 
at  20°  containing  over  75  per  cent  of  acid.  The  first  crop  of  crystals  was 
drained  as  completely  as  possible,  usually  with  centrifugal  drainage  in  a  plat- 
inum centrifugal  machine,^  and  the  product  was  twice  recrystallized.  On  ac- 
count of  the  high  solubility  of  the  acid  and  the  low  temperature  coefficient  of 
solubUity  between  0°  and  100°  the  yields  of  acid  were  very  small,  hence  the 
mother-liquors  were  worked  up  for  several  successive  yields  of  crystals.  The 
thrice  recrystaUized  acid  was  preserved  in  a  desiccator  containing  soKd  caustic 
potash.    It  was  designated  Sample  I. 

The  solutions  of  iodic  acid,  as  well  as  the  solid  substance  and  even  iodine  pent- 
oxide  made  from  the  acid,  possessed  to  a  slight  degree  a  peculiar  odor  which 
may  be  described  as  "aromatic."  This  odor  persisted  through  the  crystalhza- 
tion  although  with  continual  diminution,  but  was  not  entirely  absent  from  acid 
which  had  been  crystallized  three  times.  It  was  later  proved  that  the  odor  is 
not  due  to  the  acid  itself  because  after  a  sufficient  number  of  crystallizations 
the  odor  disappears.  Furthermore,  it  was  experimentally  proved  that  no  ap- 
preciable amount  of  iodine  was  contained  in  this  vapor,  for  when  a  concen- 
trated solution  of  the  acid  is  boiled  no  iodine  can  be  detected  in  the  distillate, 
and  when  a  current  of  air  is  conducted  over  the  solid  at  ordinary  temperatures 
into  a  solution  of  sulphur  dioxide  no  iodine  can  be  found  in  the  sulphurous  acid 
solution.  Even  at  high  temperatures  the  pentoxide  loses  in  weight  very  slowly, 
10  gm.  losing  only  i  mg.  at  240°  in  4  hours.  By  far  the  greater  part  of  this  loss 
is  due  to  decomposition  of  the  pentoxide,  for  free  iodine  can  be  easily  detected  in 
the  air  which  has  passed  over  the  substance  at  high  temperatures. 

The  possibility  of  the  presence  of  metallic  impurity  in  the  iodic  acid,  arising 
from  the  action  of  iodine  on  the  glass  of  the  flask  in  which  it  was  condensed, 
was  tested  by  evaporating  to  dryness  the  mother-liquor  of  the  last  crystalliza- 
tion of  the  original  solution  and  heating  the  residue  in  a  weighed  platinum  boat 
in  a  current  of  pure  dry  air  until  the  pentoxide  was  completely  decomposed. 
The  boat  gained  in  weight  only  0.0006  gm.,  part  of  this  gain  being  due  to  the 
formation  of  platinous  iodide.  Faint  tests  for  sodium  and  calciima  only  could 
be  obtained  with  the  residue  in  the  spectroscope. 

SILVER. 

Silver  was  purified  by  methods  already  found  in  this  laboratory  to  be  effective 
for  the  purpose  (page  22).  A  dilute  solution  of  silver  nitrate  was  first  precipi- 
tated with  hydrochloric  acid  in  excess,  and  the  precipitate  was  thoroughly 
washed  with  pure  water.  The  chloride  was  next  reduced  by  means  of  invert 
sugar  and  an  excess  of  sodivmi  hydroxide,  and  the  metal,  after  long-continued 
washing  by  decantation,  was  fused  upon  charcoal.    After  the  buttons  had  been 

^  Baxter:  Jour.  Amer.  Chem.  Soc,  30,  286  (1908). 


A  REVISION  OF  THE  ATOMIC  WEIGHTS  OF  IODINE  AND  SILVER.         121 

cleansed  by  scrubbing  with  sand  and  etching  with  nitric  acid,'they  were  dissolved 
in  nitric  acid  and  the  silver  was  precipitated  as  metal  by  msans  of  ammonium 
formate  made  from  redistilled  ammonia  and  formic  acid.  This  precipitate  in 
turn  was  thoroughly  washed  and  the  metal  was  fused  in  the  flame  of  a  carefully 
cleaned  blast  lamp  in  a  crucible  of  lime  made  from  purified  calcium  nitrate  and 
carbonate.  After  being  cleansed  as  before,  the  buttons  were  electrolyzed  and 
the  electrolytic  crystals,  after  being  washed  and  dried,  were  fused  in  a  current 
of  pure  electrolytic  hydrogen  in  an  unglazed  porcelain  boat  lined  with  the  purest 
lime.^  The  resulting  buttons  were  cut  into  fragments  of  from  3  to  6  gm.  with 
a  fine  jeweller's  saw,  etched  with  small  portions  of  dilute  nitric  acid  until  the 
acid  used  in  etching  was  free  from  iron,  and  finally  the  fragments  were  washed 
and  thoroughly  dried  in  a  vacuum  at  about  400°.  Two  different  specimens  of 
silver  were  prepared  in  essentially  the  same  way,  Samples  A  and  B. 

CONVERSION   OF   IODIC   ACID   INTO   IODINE   PENTOXIDE. 

The  conversion  of  the  iodic  acid  into  iodine  pentoxide  was  effected  by  heating 
the  substance  in  a  current  of  dry  gas.  At  the  beginning  of  the  research  the  hope 
was  entertained  that  the  final  product  might  be  subHmed.  In  no  experiment, 
however,  have  we  been  able  to  detect  the  least  trace  of  sublimation,  even  when 
the  pentoxide  was  heated  to  the  decomposition  point.  Iodic  acid  may  be  made 
to  lose  all  its  water  of  composition  without  fusion,  and  consequently  it  was  to  be 
expected  that  under  these  conditions  dehydration  could  be  made  nearly  if  not 
quite  complete.  T.  W.  Richards  has  already  pointed  out  that  a  solid  which 
loses  water  of  composition  without  fusion  is  left  in  the  form  of  a  skeleton,  from 
the  innermost  interstices  of  which  the  water  vapor  may  escape,  while,  if  fusion 
takes  place  during  drying,  a  portion  of  the  original  salt  may  be  enclosed  within 
an  impervious  coating  of  anhydrous  salt  so  that  escape  of  water  is  impossible.^ 

It  is  commonly  stated  that  the  dehydration  of  iodic  acid  takes  place  in  two 
stages,  two-thirds  the  water  being  lost  below  130°  and  the  remainder  at  about 
200°.^  The  acid  itself,  if  heated  rapidly,  melts  at  110°  with  the  separation  of 
the  solid' phase  I2O5.HIO3,  although  this  second  phase  shows  no  indication  of 
melting  up  to  the  temperature  of  the  second  stage  in  the  dehydration.  Our 
earlier  experience  was  not  in  accordance  with  the  above  statements.  The  acid, 
when  heated  to  about  110°,  lost  all  its  water  of  composition,  at  this  tempera- 
ture. This  was  shown  conclusively  in  one  experiment  by  heating  a  weighed 
amount  of  iodic  acid  for  some  time  at  about  iio°  and  then  reweighing.    The 

*  Fusion  in  hydrogen  (Baxter:  Proc.  Amer.  Acad.,  39,  249  [1903])  is  probably  safer  than 
fusion  in  a  vacuum,  since  it  eliminates  the  possibility  of  the  taking  up  of  sulphur  by  the  silver 
from  the  rubber  ring  used  in  fitting  the  hollow  brass  stoppers  into  the  porcelain  fusion  tube. 
Richards  and  Wells  have  shown  that  silver  fused  in  hydrogen  is  certainly  as  pure  as  any  other. 

2  ZeU.  physik.  Chem.,  46, 194  (1903). 

'  For  a  discussion  of  this  subject  see  Groschuff,  Zeit.  anorg.  Chem.,  47,  333  (1905). 


122  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

acid  lost  the  amount  of  water  theoretically  necessary  for  the  formation  of  the 
pentoxide,  and  further  heating  to  300°  produced  only  a  negligible  diminution 
in  weight.  On  one  later  occasion,  however,  when  a  comparatively  crude  speci- 
men of  acid  was  being  dehydrated,  the  second  phase  unexpectedly  appeared, 
and  in  all  subsequent  experiments  the  same  result  was  obtained,  no  matter 
what  the  source  of  the  iodic  acid,  or  how  carefully  the  apparatus  was  cleaned 
before  use.  This  point  is  additional  proof  that  the  second  phase  is  a  definite 
chemical  compound.  The  obvious  explanation  of  the  phenomenon  is  that  the 
laboratory  became  inoculated  with  "germs "  of  the  second  phase  so  that  its  for- 
mation was  thereafter  always  induced. 

The  iodic  acid  was  dehydrated  in  a  platinum  boat  contained  in  a  hard-glass 
tube  heated  by  a  solid  aluminimi  oven  (fig.  4,  page  78).  When  heated  with  a 
Bunsen  flame,  this  bath  could  be  regulated  in  temperature  to  within  one  or 
two  degrees  without  the  least  difficulty,  even  without  the  jacket  of  asbestos 
paper  which  was  usually  employed.  Thermometers  placed  inside  the  hard- 
glass  tube  and  in  a  cavity  in  the  blocks  registered  the  same  temperature  within 
a  degree  even  when  the  oven  was  heated  to  300°. 

At  first  the  air  was  purified  and  dried  by  passing  over  hot  copper  oxide  in  a 
hard-glass  tube,  then  through  two  towers  containing  beads  moistened  with 
silver  nitrate  and  caustic  potash  solutions,  respectively,  and  finally  through 
four  towers  containing  beads  moistened  with  concentrated  sulphuric  acid  to 
which  a  small  quantity  of  potassium  dichromate  had  been  added  in  order  to  pre- 
vent any  possibility  of  reduction.  The  apparatus  was  constructed  wholly  of 
glass  with  ground  or  fused  joints,  rubber  and  grease  being  carefully  avoided. 
When  heated  in  a  current  of  air  thus  purified  and  dried,  even  the  purest  iodine 
pentoxide  became  somewhat  brown  owing  possibly  to  liberation  of  iodine. 
The  use  of  electrolytic  oxygen  in  place  of  air  failed  to  prevent  this  phenomenon. 
Finally  it  was  found  that  if  the  air  was  dried  more  thoroughly,  by  means  of  re- 
sublimed  phosphorus  pentoxide,  the  difficulty  could  be  avoided.  The  cause  of 
this  marked  catalytic  effect  from  the  minute  quantity  of  moisture  which  is  not 
absorbed  by  sulphuric  acid  was  not  determined. 

The  hard-glass  tube  in  which  the  heating  was  conducted  formed  part  of  a 
botthng  apparatus  by  means  of  which  the  boat  could  be  transferred  to  a  weigh- 
ing-bottle without  exposure  to  moist  air  (see  fig.  i,  page  8).  Although  pre- 
cautions were  taken  to  prevent  exposure  of  the  substance  to  moisture  before 
weighing,  little  danger  was  to  be  feared  from  this  source,  for  in  air  ordinarily 
moist  the  pentoxide  absorbs  water  very  slowly.  For  instance  4.4  gm.  of  pent- 
oxide, when  exposed  in  the  platinum  boat  to  the  air  of  the  laboratory  for  2 
hours,  gained  only  0.0008  gm. 

Even  when  dried  under  the  most  favorable  conditions  the  above-mentioned 
darkening  of  the  pentoxide  took  place  if  the  temperature  rose  much  over  250°. 
Accordingly  the  temperature  was  not  allowed  to  pass  this  point  in  drying  the 
substance  for  analysis.    Furthermore,  since  it  was  by  no  means  certain  that 


A  REVISION  OF  THE  ATOMIC  WEIGHTS  OF  IODINE  AND   SILVER.         1 23 

every  trace  of  water  could  be  expelled  from  the  pentoxide  at  this  temperature, 
and  since  it  seemed  probable  that  the  amount  of  water  retained  would  vary 
with  the  temperature  and  time  of  treatment,  the  conditions  in  the  different 
experiments  were  made  as  nearly  as  possible  identical. 

The  details  of  manipulation  were  as  follows:  The  crystals  of  iodic  acid  were 
powdered  in  a  new  smooth  agate  mortar  together  with  a  small  quantity,  about 
2  per  cent  of  the  weight  of  the  acid,  of  the  phase  produced  in  the  first  stage  in  the 
dehydration.  This  step  was  of  advantage  in  catalyzing  the  first  stage  in,  the 
dehydration,  for  frequently,  when  the  precaution  of  thus  inoculating  the  iodic 
acid  was  omitted,  the  temperature  reached  130°  before  dehydration  began, 
whereas  when  this  precaution  is  taken  the  water  first  appears  as  low  as  85°, 
Since  th^  iodic  acid  itself  melts  at  110°  when  in  contact  with  the  phase  pro- 
duced at  this  temperature,  it  seemed  better  to  prevent  the  acid  from  reaching 
this  temperature  until  the  first  stage  in  the  dehydration  was  past.  As  a  matter 
of  fact,  preliminary  water  determinations  in  the  dried  material  indicated  a 
sHghtly  larger  proportion  of  water  in  acid  which  had  not  been  inoculated  with 
the  second  phase. 

The  weighed  boat,  containing  from  6  to  10  gm.  of  the  iodic  acid,  was  next 
heated  in  a  slow  current  of  air  at  90°  to  1 10°  until  the  first  two  thirds  of  the  water 
of  composition  had  been  given  ofi  by  the  iodic  acid  and  had  been  expelled  from 
the  tube  by  the  current  of  dry  air.  The  temperature  was  then  raised  until  the 
second  portion  of  water  was  given  oS.  Usually  the  pentoxide  began  to  form 
at  about  220°.  There  is  no  evidence  of  fusion  at  this  point,  even  if  the  tempera- 
ture rises  much  above  220°  before  all  the  remaining  water  is  expelled.  After 
all  the  water  had  disappeared  the  temperature  was  raised  to  240°  and  main- 
tained at  this  point  for  4  hours.  Then  the  boat  was  transferred  to  the  weigh- 
ing-bottle, allowed  to  stand  in  the  desiccator  with  the  counterpoise  for  some 
time,  and  was  weighed. 

DETERMINATION    OF   IODINE   IN   IODINE   PENTOXIDE. 

The  analysis  of  the  iodine  pentoxide  for  iodine  was  effected  by  dissolving  the 
substance  in  water,  reducing  the  iodic  acid  solution  to  hydriodic  acid  with  a 
suitable  reducing  agent,  and  titrating  the  hydriodic  acid  against  a  weighed 
amount  of  silver. 

The  operation  in  detail  was  as  follows:  After  being  weighed,  the  boat  with 
its  contents  was  placed  in  a  large  thick-walled  flask  and  immediately  covered 
with  about  500  c.c.  of  the  purest  water,  in  which  it  slowly  dissolved.  Even  when 
the  pentoxide  was  slightly  colored  the  solution  was  colorless  to  the  eye  and  abso- 
lutely clear.  The  weighing-bottle  was  rinsed  with  water  and  the  rinsings  were 
added  to  the  main  solution  in  the  flask.  In  a  number  of  early  experiments  the 
reduction  was  accompHshed  by  slowly  pouring  the  dilute  solution  of  iodic  acid 
into  a  solution  of  a  sHght  excess  of  sulphurous  acid.  Very  nearly  the  theoretical 
amoimt  of  the  purest  silver  was  weighed  out  and  dissolved  in  redistilled 


124  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

nitric  acid  diluted  with  an  equal  volume  of  water  as  described  on  page  12. 
After  the  silver  was  completely  dissolved,  the  solution  was  diluted  with  an 
equal  volume  of  water  and  heated  until  free  from  nitrous  acid.  This  latter 
precaution  was  especially  necessary,  since  nitrous  acid  readily  liberates  iodine 
from  iodides,  while  nitric  acid  has  this  effect  only  when  hot  or  rather  concen- 
trated. Finally  the  solution  was  diluted  until  at  least  as  dilute  as  thirtieth 
normal  and  was  added  with  constant  agitation  to  the  solution  of  the  hydriodic 
acid  which  had  been  diluted  to  a  similar  concentration  in  a  large  glass-stoppered 
precipitating  bottle.  The  whole  was  then  thoroughly  shaken  and  allowed  to 
stand  until  clear.  In  order  to  determine  whether  silver  or  iodide  was  in  excess, 
30  c.c.  portions  of  the  solution  were  tested  with  silver  nitrate  and  hydriodic  acid 
in  the  nephelometer.  The  results  of  these  earlier  analyses  were  extremely  un- 
satisfactory, the  end-point  changing  continuously  with  time,  and  an  unreason- 
ably large  amount  of  silver  was  required.  This  could  not  have  been  due  to 
occlusion  of  silver  nitrate  by  the  silver  iodide,  since  it  has  already  been  shown 
by  one  of  us  that  at  concentrations  less  than  thirtieth  normal,  if  very  nearly 
equivalent  amounts  of  silver  and  iodide  are  used,  the  latter  diflSculty  is  too 
small  to  have  an  appreciable  effect.^ 

The  cause  of  the  trouble  was  finally  traced  to  occlusion  of  silver  sulphate  by 
the  silver  iodide.  Richards  and  Jones  ^  found  that  silver  chloride  occludes  silver 
sulphate  very  markedly  and  tenaciously,  hence  it  is  not  in  the  least  surprising 
to  find  the  same  difficulty  here.  It  is  to  be  noted  that  the  concentration  of  sul- 
phuric acid  in  these  experiments  is  relatively  large,  three  molecules  of  sulphuric 
acid  being  produced  in  the  reduction  for  every  atom  of  iodine.  That  this  diffi- 
culty did  not  appear  in  the  above  mentioned  researches  upon  the  ratio  of  iodine 
to  silver,  where  iodine  was  reduced  with  sulphurous  acid  and  the  hydriodic  acid 
titrated  against  silver,  is  undoubtedly  due  to  the  fact  that  the  concentration  of 
sulphuric  acid  was  relatively  much  lower,  only  one  molecule  of  sulphuric  acid 
being  formed  in  the  reduction  of  one  molecule  of  iodine. 

A  search  for  a  more  satisfactory  reducing  agent  than  sulphurous  acid  failed 
to  reveal  any  substance  more  promising  than  hydrazine,  for  the  reduction  of 
iodic  acid  with  this  substance  yields  only  hydriodic  acid,  nitrogen  and  water.^ 
In  order  to  avoid  the  introduction  of  undesirable  acids,  the  hydrazine  was  used 
in  the  form  of  a  solution  of  the  hydroxide  instead  of  as  a  salt.  The  hydrazine 
hydroxide  was  made  by  distilling  either  the  chloride  or  the  sulphate  with  a 
concentrated  solution  of  a  considerable  excess  of  sodium  hydroxide  in  a  plat- 
inum still.    The  product  was  then  redistilled  in  the  platiniun  still  to  eliminate 

*  Baxter:  Proc.  Amer.  Acad.,  41,  77  (1905);  Jour.  Amer.  Chem.  Soc,  27,  880;  Zeit.  anorg. 
Chem.,  46,  41.    (See  page  107.) 

2  Pub.  Carnegie  Institution,  69,  73  (1907);  Jour.  Amer.  Chem.  Soc,  29,  837;  Zeit.  anorg. 
Chem.,  55,  84. 

^  Brown  and  Shetterly  have  shown  that  no  hydronitric  acid  is  formed  by  the  action  of 
hydrazine  on  iodatcs  or  iodine.   Jour.  Amer.  Chem.  Soc.,  30,  53  (1908) 


A  REVISION  OF  THE   ATOMIC  WEIGHTS  OF  IODINE   AND  SILVER. 


125 


possible  traces  of  chlorides,  and  was  preserved  in  a  platinum  flask.  Even  when 
made  from  the  chloride,  the  doubly  distilled  product  was  absolutely  free  from 
chlorides.  The  approximate  concentration  of  the  solution  was  determined 
shortly  before  use  by  titration  against  a  standard  solution  of  iodic  acid  or 
potassium  permanganate.  Preliminary  experiments  showed  that  there  is  no 
danger  of  the  reduction  of  the  silver  salts  by  a  slight  excess 
of  hydrazine  in  nitric  acid  solution. 

Special  pains  were  taken  in  the  reduction  of  the  iodic  acid 
with  hydrazine  to  avoid  any  possibility  of  loss  of  iodine  by 
volatilization.  The  solution  of  the  iodic  acid  was  trans- 
ferred to  an  8-liter  bottle  with  a  carefully  ground  stopper, 
and,  after  the  iodic  acid  had  been  neutralized  with  ammonia, 
very  slightly  less  than  the  theoretical  quantity  of  hydrazine 
hydroxide  was  added.  Since  in  alkaline  solution  hydrazine 
is  without  effect  upon  iodic  acid,  the  solution  was  next 
made  acid  by  slow  addition  of  nitric  acid. 

As  soon  as  the  neutral  point  has  been  passed,  iodine  is 
freed  by  interaction  of  the  hydriodic  acid  with  the  iodic 
acid.  Lest  any  of  this  iodine  be  volatilized  from  the  solu- 
tion before  it  could  be  reduced  by  the  hydrazine  the  nitric 
acid  was  introduced  at  the  bottom  of  the  solution  through  a 
long  funnel.  Thus  iodine  was  sure  to  be  completely  re- 
duced before  reaching  the  surface  of  the  solution.  A  specially 
devised  stopper,  shown  in  the  accompanying  diagram,  pro- 
vided for  the  escape  of  the  nitrogen  through  a  long  col- 
imin  of  bulbs,  which  served  to  catch  spatterings  or  soluble 
vapors,  if  any  reached  this  point.  As  a  further  precaution 
against  the  escape  of  iodine  a  small  quantity  of  sulphurous 
acid  was  poured  into  the  bottle  through  the  train  of  bulbs, 
acid  served  also  to  complete  the  reduction  of  the  iodic  acid 


Fig.  5.  —  Appara- 
tus for  reducing 
iodic  acid. 


This  sulphurous 
The  amount  of 

sulphuric  acid  formed  in  this  way  was  not  sufficient,  however,  to  produce 
appreciable  occlusion  of  silver  sulphate. 

After  the  reduction  and  subsequent  dilution  of  the  solution  to  a  concentra- 
tion between  thirtieth  and  fiftieth  normal,  the  hydriodic  acid  was  precipitated 
with  a  dilute  solution  of  a  weighed  amount  of  silver  as  before  described.  On 
account  of  the  large  bulk  of  the  mother-liquor,  6  to  7  liters,  the  end-point  as 
determined  in  the  nephelometer  was  somewhat  uncertain,  especially  since  it 
seemed  to  be  complicated  by  a  minute  trace  of  silver  iodide  held  in  suspension. 
Hence,  instead  of  attempting  to  use  exactly  the  theoretical  quantity  of  silver,  an 
amount  was  employed  a  few  tenths  of  a  milligram  in  excess  of  the  required  quan- 
tity. Then  this  excess  was  determined  by  evaporating  the  supernatant  liquid 
to  a  very  small  bulk,  precipitating  the  excess  of  silver  as  silver  iodide  and  deter- 
mining the  silver  iodide  either  gravimetrically  upon  a  Gooch-Munroe-Neubauer 


126  RESEARCHES    UPON   ATOMIC   WEIGHTS. 

crucible,  or  nephelometrically  by  comparison  with  dilute  standard  solutions 
of  silver. 

Before  evaporation,  the  supernatant  solution  was  carefully  filtered  through 
the  Neubauer  crucible  and  the  precipitate  of  silver  iodide  was  washed  by  de- 
cantation  several  times  with  pure  water  to  remove  any  adsorbed  silver  nitrate. 
That  washing  was  able  to  remove  any  adsorbed  silver  nitrate  was  shown  by 
treating  a  very  carefully  washed  precipitate  of  silver  iodide  resulting  from  one 
of  the  analyses  with  a  solution  of  silver  containing  a  few  tenths  of  a  milligram 
of  silver  in  6  liters.  After  thorough  agitation  with  the  precipitate  the  solution 
was  filtered  and  evaporated  as  in  the  analyses,  after  washing  of  the  precipi- 
tate. Practically  all  this  silver  was  found  in  the  evaporated  solution.  Although 
the  silver  determined  in  this  way  includes  the  silver  iodide  dissolved  in  the 
mother-liquor,  the  error  introduced  is  very  small,  the  solubility  of  silver  iodide 
being  probably  at  least  as  low  as  0.0000035  S"^-  V^^  Hter.^  The  platinum  boat 
was  not  appreciably  changed  in  weight. 

DETERMINATION    OF   MOISTURE   IN   IODINE   PENTOXIDE. 

The  determination  of  the  water  content  of  the  iodine  pentoxide  when  dried 
under  the  conditions  used  in  the  analyses  for  iodine,  was  fully  as  difficult  as 
the  iodine  determination,  and  it  was  in  preliminary  attempts  to  determine  the 
water  content  that  evidence  was  obtained  which  led  to  the  adoption  of  the 
above  precautions  for  reducing  the  water  content  to  a  constant  low  value.  The 
method  of  operation  consisted  in  brief  in  completely  decomposing  the  pent- 
oxide  into  iodine  and  oxygen  by  heating  in  a  current  of  dry  air,  and,  after  re- 
moving the  iodine  as  far  as  possible  by  condensation  and  finally  by  a  layer  of  hot 
silver,  collecting  the  water  in  a  weighed  phosphorus  pentoxide  tube.  Great 
pains  were  taken  that  the  iodine  pentoxide  should  be  prepared  for  the  water 
determinations  exactly  as  for  iodine  determinations.  In  order  to  avoid  absorp- 
tion of  water  between  the  heating  and  the  decomposition,  the  decomposition  of 
the  pentoxide  took  place  immediately  upon  the  completion  of  the  heating  with- 
out interruption  of  the  experiment,  the  weighing  of  the  pentoxide,  which  need 
not  be  very  accurate  on  account  of  the  low  per  cent  of  water,  having  taken  place 
before  the  long  heating  at  240°. 

The  iodic  acid  used  in  the  water  determinations  was  not  made  from  either 
purified  iodine  or  purified  nitric  acid,  and  most  of  it  was  prepared  in  glass  ves- 
sels. However,  it  was  very  carefully  purified  by  repeated  crystallization  from 
aqueous  solution  in  platjinum  vessels  with  centrifugal  drainage  of  the  crystals 
in  each  case,  and  gave  every  outward  evidence  of  great  purity.  Not  only  were 
the  final  crystals  pvire  white,  but  the  final  mother-liquor  was  colorless.  The 
crystals  gave  a  perfectly  clear  solution  in  water  and  left  no  appreciable  residue 
when  decomposed  in  the  platinum  boat. 

1  Kohlrausch,  Zeit.  physik.  Chem.,  50,  355  (1904). 


A  REVISION   OF   THE   ATOMIC   WEIGHTS   OF   IODINE   AND    SILVER.  1 27 

The  platinum  boat,  containing  about  25  gm.  of  acid  which  had  been  inocu- 
lated with  the  second  phase,  was  freed  from  water  by  heating  at  100°,  and  then 
at  220°  +  in  the  usual  way.  It  was  then  bottled  and  weighed.  Next  the  boat 
was  transferred  to  a  long  hard-glass  tube  very  carefully  ground  into  the  socket 
of  the  botthng  apparatus  and  was  heated  to  340°  for  4  hours  in  a  current  of  dry 
air.  Toward  the  end  of  the  heating  all  of  the  apparatus  beyond  the  phosphorus 
pentoxide  drying  tube  was  gently  heated  with  a  Bunsen  flame  in  order  to  dis- 
lodge any  adsorbed  water  from  the  inside  surface  of  the  glass.  Although  by 
far  the  greater  part  of  the  iodine  formed  in  the  decomposition  condensed  in  the 
hard-glass  tube,  a  small  quantity  of  iodine  vapor  was  always  carried  along  by 
the  current  of  gases.^  For  the  absorption  of  this  iodine  vapor  a  small  hard-glass 
tube  containing  small  electrolytic  crystals  of  silver  which  had  been  dried  by 
heating  to  about  400°  in  a  vacuum  was  carefully  ground  on  the  end  of  the  first 
tube.  This  silver  tube  was  attached  during  the  decomposition  of  the  pentoxide 
and  was  heated  to  very  dull  redness.  The  column  of  metallic  silver  was  several 
inches  in  length,  and  although  considerable  silver  iodide  was  produced  at  the 
end  of  the  colvunn  nearest  the  decomposition  tube,  the  silver  at  the  other 
end  of  the  column  remained  perfectly  bright  through  all  the  determinations, 
showing  the  absorption  of  iodine  to  have  been  complete. 

The  U -tubes  for  the  absorption  of  water  were  provided  with  glass  stopcocks 
and  were  filled  with  phosphorus  pentoxide  which  had  been  freshly  sublimed  in 
a  current  of  oxygen.  They  were  weighed  by  substitution,  with  the  use  of  a 
similar  tube  as  counterpoise.  Before  being  weighed,  the  tubes  were  wiped  with 
a  damp  cloth  and  were  allowed  to  stand  near  the  balance  case  for  at  least  an 
hour.  The  tubes  were  weighed  with  one  stopcock  open.  The  balance  was  pro- 
vided with  a  few  milligrams  of  radium  bromide  of  radio-activity  loooo  to  dispel 
electrical  charges.  Under  these  conditions  no  difficulty  was  experienced  in 
weighing  the  tubes  within  a  few  hundredths  of  a  milligram,  since  they  quickly 
came  to  constancy  in  the  balance  case  and  retained  their  weights  unchanged 
for  days  at  a  time.  Two  phosphorus  pentoxide  tubes  were  used  in  the  first  ex- 
periments, but  since  the  gain  in  weight  of  the  second  tube  was  found  to  be  neg- 
ligible in  all  cases  where  it  was  used,  the  second  tube  was  omitted  in  the  later 
experiments.  Still  another  phosphorus  pentoxide  tube  was  placed  beyond  the 
weighed  tubes  as  a  protection  against  the  back  diffusion  of  moisture.  Blank 
experiments  were  usually  run  after  the  collection  of  the  water  resultirig  from  the 
decomposition.  Frequently  no  gain  in  weight  of  the  tube  in  these  blank  ex- 
periments could  be  detected  and  in  no  case  did  the  gain  ill  weight  amount  to 
more  than  o.i  mg. 

The  decomposition  of  the  iodine  pentoxide  was  effected  by  removing  the  alumi- 
num oven  and  heating  the  tube  to  the  temperature  of  decomposition  of  the 

'  Baxter,  Hickey,  and  Holmes,  "  The  Vapor  Pressure  of  Iodine,"  Jour.  Amer.  Chem.  Soc, 
39,  127  (1907). 


128 


RESEARCHES  UPON   ATOMIC  WEIGHTS. 


pentoxide,  about  350°,  with  a  Bunsen  burner.  Decomposition  was  conducted 
as  evenly  as  possible  so  as  to  avoid  an  unduly  rapid  current  of  oxygen  from  the 
apparatus  with  consequent  incomplete  absorption  of  the  water.  When  decom- 
position was  complete  the  condensed  iodine  was  heated  above  its  melting-point 
to  set  free  traces  of  absorbed  moisture.  Then  the  decomposition  tube  was  swept 
out  with  a  current  of  dry  air  and  the  absorption  tube  was  weighed.  In  four 
experiments  carried  out  as  above  the  following  results  were  obtained: 


No  of 
analysis. 

Weight  of 
I2OS. 

Weight  of  H2O. 

Per  cent  of 
H2O. 

I 
2 

3 

4 

gm. 

25.0 

20.7 

26.1 

24.6 

gm. 
0.00060 
0.00032 
0.00067 
0.00060 

0.0024 
0.0016 
0.0026 
0.0024 

Average    .   .   . 

0.0023 

In  addition,  several  experiments  were  performed  in  order  to  determine  the 
effect  of  varying  the  conditions  of  treatment.  In  analysis  5  the  heating  at  240" 
lasted  only  one  hour,  and  in  analysis  6  the  iodine  pentoxide  was  heated  for  four 
hours  at  260°.  In  analyses  7  to  11  only  a  very  small  quantity  of  the  second 
phase  was  used  to  inoculate  the  pentoxide;  in  analyses  7  and  8  the  substance 
was  powdered  to  the  same  degree  of  fineness  as  in  the  first  set  of  experiments;  in 
analysis  9  the  material  was  very  finely  powdered,  and  in  analyses  10  and  11  the 
material  was  rather  coarsely  powdered. 


No.  of 

Weight  of 

Weight  of 

Per  cent 

analysis. 

I2O6. 

H2O. 

of  H2O. 

gm. 

gm. 

5 

2S-S 

0.00066 

0.0026 

6 

25-3 

0.00062 

0.0025 

7 

24.9 

0.00113 

0.0045 

8 

26.8 

O.OOIOO 

0.0037 

9 

18.9 

0.00077 

0.0041 

10 

21.3 

0.00133 

0.0062 

II 

25.0 

0.0015s 

0.0062 

The  differences  in  composition  of  the  pentoxide  actually  observed,  even  with 
very  widely  differing  methods  of  treatment,  are  so  small  that  there  can  be  no 
doubt  that  the  slight  variations  in  treatment  likely  to  occur  in  the  course  of  an 
analysis  could  not  have  had  an  appreciable  effect. 


A  REVISION  OF   THE  ATOMIC  WEIGHTS   OF   IODINE  AND   SILVER. 


129 


SPECIFIC   GRAVITY   OF   IODINE    PENTOXIDE. 

In  order  to  find  out  exactly  the  buoyant  effect  of  air  upon  the  weight  of  the 
pentoxide  an  accurate  knowledge  of  the  specific  gravity  of  the  substance  was 
necessary.  The  density  was  determined  by  displacement  of  kerosene  of  known 
specific  gravity,  by  pentoxide  which  had  been  prepared  as  previously  described 
by  heating  in  a  small  platinum  boat.  The  boat  was  weighed  in  a  small  weighing- 
bottle,  and  iromediately  after  the  weighing  the  boat  was  covered  with  kerosene 
in  the  bottle.  The  bottle  was  placed  in  a  small  vacuum  desiccator  and  the 
desiccator  was  kept  exhausted  with  continual  jarring  until  apparently  every 
trace  of  air  had  been  displaced  from  the  powder.  A  special  pycnometer  stopper  ^ 
was  next  inserted  in  the  bottle  and  the  pycnometer  was  set  while  immersed  in  a 
water  bath  at  25°  C. 

The  Specific  Gravity  of  Iodine  Pentoxide. 
Specific  Gravity  of  Kerosene  =  0.7655. 


Weight  of  IjOb 
in  vacuum. 

Weight  of  dis- 
placed kerosene 
in  vacuum. 

Specific  gravity 
25%°*' 

gm. 

5-5136 
5-2704 

gm. 
0.8811 
0.8393 

4.790 
4.807 

Aver. 

lee      4.700^ 

The  vacuimi  correction  for  one  apparent  gram  of  iodine  pentoxide  calcu- 
lated from  the  above  value  for  the  density  is  +0.000106  gm.,  the  weights  being 
assimied  to  have  the  density  8.3.'  A  vacuum  correction  of  —0.000031  was 
applied  for  every  gram  of  silver. 

A  nearly  new  No.  10  Troemner  balance  was  used  in  all  the  weighing.  It  was 
readily  sensitive  to  0.2  mg.  The  weights  were  carefully  standardized  to  hun- 
dredths of  a  milligram,  by  the  method  described  by  Richards.* 

*  For  details  of  pycnometer  and  setting  see  Baxter  and  Hines,  Amer.  Chem.  Jour.,  31,  220 
(1904). 

*  At  0°  Ditte  obtained  the  value  4.487.  At  9°  Kammerer  obtained  the  value  4.799.  Filhol 
found  the  density  to  be  4.250.   Dammer.  Handb.  der  anorg.  Chem.,  i,  560. 

^  See  page  40. 

*  Jour.  Amer.  Chem.  Sac,  22, 144  (1900). 


130  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

ADSORPTION    OF   AIR   BY   IODINE    PENTOXIDE. 

Since  the  pentoxide  is  formed  by  a  process  of  double  efflorescence,  it  must  be 
extremely  porous,  with  consequently  an  unusually  large  surface  with  relation 
to  its  weight.  Hence  it  might  be  supposed  that  such  a  substance  could  adsorb 
appreciable  amounts  of  gases,  possibly  even  air.  This  question  was  investi- 
gated with  iodine  pentoxide  by  determining  the  difference  in  weight  of  the  sub- 
stance in  a  vacuum  and  in  air. 

Two  weighing-bottles  were  constructed  with  long,  very  well  ground  stoppers 
which  terminated  in  stopcocks  through  which  the  tubes  could  be  exhausted. 
These  tubes  were  very  closely  of  the  same  weight  and  of  very  nearly  the  same 
internal  capacity.  The  tubes  were  first  exhausted  and  compared  in  weight  by 
substitution.  Next  they  were  filled  with  dry  air  and  again  weighed,  the  weigh- 
ings being  carried  out  with  the  stopcocks  open.  Both  steps  were  then  repeated 
with  essentially  the  same  results. 

Into  one  of  the  tubes  was  introduced  about  25  gm.  of  carefully  dried  pentoxide 
and  both  tubes  were  completely  exhausted.  When  the  tube  containing  the  pen- 
toxide was  warmed  to  about  150°  no  perceptible  quantity  of  gas  was  evolved. 
After  the  difference  in  weight  of  the  exhausted  tubes  had  been  determined,  they 
were  again  filled  with  dry  air  and  weighed,  and  the  process  of  exhausting  the 
tubes  and  filUng  them  with  air  was  repeated.  In  all  the  weighings  the  tubes 
were  treated  as  described  in  the  case  of  the  phosphorus  pentoxide  tubes. 

gm. 

I.  Difference  in  weight  of  exhausted  tubes 0.01405 

I.  Difference  in  weight  of  tubes  filled  with  air 0.01395 

I.  Difference  in  air  content  of  tubes     o.oooio 

II.  Difference  in  weight  of  exhausted  tubes 0.01415 

II.  Difference  in  weight  of  tubes  filled  with  air 0.01397 

II.  Difference  in  air  content  of  tubes     0.00018 

Average  difference  in  air  content  of  tubes 0.00014 

Weight  of  tube  with  iodine  pentoxide 58.6353 

Weight  of  tube 32.7966 

Weight  of  iodine  pentoxide 25.8387 

I.  Difference  in  weight  of  exhausted  tubes,  one  containing  iodine  pentoxide  25.85120 
I.  Difference  in  weight  of  tubes  filled  with  air  at  19°  C.  and  758  mm.   .    .  25.84473 

Difference 0.00647 

Average  difference  in  air  content  of  empty  tubes 0.00014 

Air  displaced  by  IjOe  at  19°  and  758  mm 0.00633 

II.  Difference  in  weight  of  exhausted  tubes,  one  containing  iodine  pentoxide  25.85091 
II.  Difference  in  weight  of  tubes  filled  with  air  at  20°  C.  and  768  mm.    .    .    .  25.84430 

Difference 0.00661 

Average  difference  in  weight  of  empty  tubes 0.00014 

Air  displaced  by  laOj  at  20°  and  768  mm 0.00647 

The  same  corrected  to  19°  and  758  mm 0.00641 

Average  weight  of  air  displaced  by  I2O5  at  19°  and  758  mm 0.00637 

Weight  of  air  displaced  by  laOs  at  19°  and  758  mm.  calculated  from  the 

density  4.799 0.00649 


A  REVISION   OF   THE   ATOMIC   WEIGHTS    OF   IODINE   AND    SILVER. 


131 


The  agreement  in  the  air  displaced  as  determined  experimentally  and  as 
calculated  from  the  observed  density  is  as  close  as  could  be  expected,  showing 
that  the  vacuum  correction  determined  on  page  1 29  is  correct.  It  is  to  be  noted 
that  if  air  is  adsorbed  by  the  powder  the  observed  weight  of  air  displaced  would 
be  less  than  that  calculated  from  the  density.  This  seems  to  be  actually  the 
case  to  a  very  sUght  degree. 

The  Ratio  of  Silver  to  Iodine  Pentoxide. 

Series  I.     By  G.  S.  Tilley. 

Sample  I  of  Iodic  Acid  and  Sample  B  of  Silver  were  Used. 


No.  of 
analysis. 

Corrected 

weight  of  IjOe 

in  vacuum. 

Weight  of 
Ag  in  vacuum. 

Weight 
of  Agl. 

Corrected 
weight  of  Ag 
in  vacuum. 

Ratio 
zAgJjOs. 

gm. 

am. 

gm. 

gm. 

li 

(  6.06570 
\  9.48035 

3.92046 

0.00042 

3.92027  1 
6.12611  f 

0.646234 

6.12703 

0.00200 

2 

7.73052 

4-99599 

0.00077 

4.99564 

0.646223 

3 

12.63909 

8.16804 

0.00058 

8.16777 

0.646231 

4 

9-49913 

6.13884 

0.00094 

6.13841 

0.646208 

5 

8-34369 

5.39206 

0.00008 

5.39202 

0.646239 

6 

8.83ISS 

5.70748 

0.00071 

5-70715 

0.646223 

7 

6.77487 

4-37836 

0.00071 

4-37803 

0.646216 

Average     .   . 

.    .     0.646221; 

The  atomic  weights  of  iodine  and  silver  calculated  from  the  ratio  of  silver  to 
pentoxide  depend,  of  course,  upon  the  ratio  of  the  atomic  weights  of  silver  and 
iodine.  The  latter  ratio  has  recently  been  subjected  to  careful  revision  by  one 
of  us,2  and  has  been  found  to  have  the  value  0.849943.  Using  this  ratio  the 
atomic  weight  of  silver  calculated  from  the  above  data  is  107.847. 

This  value  is  highly  sensitive  to  changes  in  the  ratio  of  silver  to  iodine,  a  posi- 
tive error  of  o.oi  per  cent  producing  a  negative  error  of  0.03  per  cent  in  the 
atomic  weight  of  silver.  Hence,  if  the  above  value  of  the  atomic  weight  of  silver 
is  too  low,  it  might  be  expected  that  the  ratio  of  silver  to  iodine  is  too  high. 
From  Baxter's  results  it  seems  certain  that  the  ratio  of  silver  to  iodine  is  at  any 
rate  no  higher  than  the  value  used  in  our  calculations. 

The  result  of  the  experiments  which  have  been  described  is  unexpected.  For 
although  several  recent  investigations  have  shown  that  the  atomic  weight  of 
silver  may  be  possibly  as  low  as  107.87,  referred  to  oxygen  16.000,  no  evidence 
has  yet  been  published  which  indicated  a  value  as  low  as  the  foregoing.  Ac- 
cordingly a  new  series  of  experiments  was  undertaken,  similar  to  the  early  one, 
with  even  greater  pains  to  avoid  possible  sources  of  error. 

^  The  first  two  analyses  were  inadvertently  mixed,  and  hence  are  combined  in  the  table. 
2  Baxter:  Proc.  Amer.  Acad.,  40,  419  (1904);  41,  73  (1905);    Jour.  Amer.  Chetn.  Soc,  26, 
1577;  27,  876;  Zeit.  anorg.  Chem.,  43,  14;  46,  36.    See  previous  paper. 


132  RESEARCHES  UPON  ATOMIC   WEIGHTS. 


PURIFICATION    OF    IODIC    ACID    AND    SILVER    FOR    THE    SECOND 
SERIES   OF   ANALYSES. 

Two  new  specimens  of  iodic  acid  were  prepared.  In  order  to  determine 
whether  crystallization  of  the  iodic  acid  alone  was  sufficient  purification,  Sample 
II  was  made  from  iodine  which  had  been  purified  by  only  a  single  distillation 
from  solution  in  an  iodide,  and  then  by  one  from  water,  while  the  nitric  acid 
was  once  distilled  through  a  quartz  condenser  with  no  special  attempt  to  remove 
chlorine.  A  portion  of  the  iodic  acid  was  prepared  in  the  transparent  fused 
quartz  flask  as  already  described.  The  remainder  was  made  in  a  large  opaque 
quartz  dish  nearly  one  liter  in  capacity.  The  inside  of  this  dish  had  been  abraded 
with  sand  to  break  the  edges  of  the  bubbles,  and  had  been  digested  for  hours  with 
acid  solutions  to  remove  soluble  impurities.  The  combined  specimens  of  acid 
were  crystallized  at  least  ten  times  in  dishes  of  transparent  fused  quartz,  with 
centrifugal  drainage,  until  free  from  the  organic  odor  previously  mentioned. 
In  the  preparation  of  this  sample  the  solutions  were  always  heated  and  evap- 
orated upon  an  electric  stove  under  a  large  Victor  Meyer  funnel,  instead  of  in 
the  special  evaporating  apparatus  previously  described. 

The  second  specimen  of  acid  was  prepared  from  iodine  resulting  from  the 
decomposition  of  the  iodine  pentoxide  in  the  water  determinations.  It  was  first 
dissolved  in  pure  sulphurous  acid  and  then  set  free  from  solution  by  distilla- 
tion with  recrystallized  potassitmi permanganate,  a  Uttleless  permanganate  than 
was  necessary  to  set  free  all  the  iodine  being  employed.  The  product  was  thus 
distilled  once  from  a  dilute  solution  of  an  iodide.  Finally  the  iodine  was  once 
distilled  from  pure  water.  The  nitric  acid  for  this  preparation  was  carefully 
freed  from  chlorine  by  double  distillation  with  a  quartz  condenser  as  previously 
described.  The  iodine  was  converted  into  iodic  acid  by  treatment  with  the 
nitric  acid  in  the  large  opaque  quartz  dish  used  for  the  preparation  of  Sample 
II.  Before  the  crystallization  of  the  iodic  acid  was  commenced,  the  powder  re- 
sulting from  the  treatment  of  the  iodine  with  nitric  acid  was  drained  and  heated 
on  the  electric  stove  until  apparently  all  nitric  acid  had  been  expelled.  Then  it 
was  heated  to  300°  in  a  current  of  pure  dry  air  in  small  portions  in  a  platinum 
boat.  The  resulting  iodine  pentoxide  was  dissolved  in  water  and  at  least  ten 
times  recrystallized  in  quartz  dishes  from  solution  in  the  purest  water,  with  cen- 
trifugal drainage  in  platinum  Gooch  crucibles.  In  spite  of  the  drastic  treatment 
to  which  the  iodine  had  been  subjected  before  conversion  into  iodic  acid,  the 
mother-liquors  of  the  first  crystallization  were  by  no  means  free  from  the 
organic  odor  previously  observed,  although  nitric  acid  seemed  to  have  been 
completely  removed.  This  odor  disappeared  gradually  during  crystalHzation  as 
before,  and  the  final  product  was  free  from  odor.  This  specimen  is  designated 
Sample  III. 


A  REVISION   OF   THE   ATOMIC  WEIGHTS   OF  IODINE   AND   SILVER.         I33 

Two  new  specimens  of  silver  were  employed.  A  portion  of  one  had  already 
been  used  in  an  investigation  upon  the  atomic  weight  of  lead.^  This  sample  was 
precipitated  once  as  silver  chloride,  once  as  metal  by  ammonium  formate, 
and  was  finally  electrolyzed.  It  is  designated  Sample  C.  Sample  D  was 
employed  in  an  investigation  upon  the  atomic  weight  of  bromine.^  It  was 
first  purified  in  part  by  precipitation  as  chloride,  in  part  by  precipitation  with 
ammonium  formate.  The  combined  material  was  then  subjected  to  two  elec- 
trolyses.   Both  specimens  were  finally  fused  in  a  current  of  pure  hydrogen. 

METHOD   OF  ANALYSES. 

The  acid  was  converted  into  pentoxide  exactly  as  in  the  first  series.  Then, 
after  weighing,  it  was  dissolved  and  reduced  with  a  slight  excess  of  hydrazine 
as  before,  and  precipitated  with  a  slight  excess  of  silver.  After  filtration  and 
evaporation  of  the  filtrate  the  excess  of  silver  was  determined  gravimetrically 
as  silver  iodide  in  a  Gooch-Munroe-Neubauer  crucible. 

Essentially  no  change  was  made  in  the  method  of  analysis  in  analyses  8  and 
9  except  that  a  slight  excess  of  hydrazine  was  employed.  At  this  point  a  tube 
50  cm.  in  length  filled  with  glass  pearls  was  substituted  for  the  column  of  bulbs 
in  the  reduction  apparatus,  since  it  was  feared  that  traces  of  the  spray  produced 
by  the  effervescence  during  reduction  might  have  been  carried  through  the 
bulbs  by  the  current  of  nitrogen  disengaged.  The  pearls  were  moistened  with 
sulphurous  acid  before  reduction  was  commenced.  This  change  in  the  appa- 
ratus was  without  effect,  the  succeeding  five  analyses  giving  results  identical 
with  previous  ones.  In  analyses  15  to  17  the  method  of  precipitation  was  re- 
versed by  pouring  the  iodide  solution  into  the  silver  nitrate  solution.  This 
change  also  was  without  effect,  hence  it  is  reasonably  certain  that  the  dilution 
of  the  solutions  during  precipitation  was  sufficient  to  prevent  occlusion  of  per- 
ceptible amounts  of  either  iodide  or  silver  nitrate.  It  is  worth  pointing  out  that 
occlusion  of  iodide  would  lower  the  observed  atomic  weight  of  silver,  while 
occlusion  of  silver  nitrate  would  produce  the  opposite  effect.  The  occlusion  of 
ammonium  nitrate  by  the  silver  iodide  would  of  course  be  without  influence 
upon  the  final  result. 

In  one  analysis,  which  is  not  recorded  in  the  table,  about  one  quarter  of  the 
iodic  acid  was  reduced  by  means  of  sulphurous  acid,  the  ratio  of  silver  to  iodine 
pentoxide  in  this  experiment  being  somewhat  higher  than  in  the  other  cases, 
owing  doubtless  to  the  occlusion  of  silver  sulphate.  In  another  analysis  the 
attempt  was  made  to  precipitate  the  silver  iodide  from  ammoniacal  solution  by 
adding  the  silver  nitrate  solution  to  the  slightly  ammoniacal  solution  of  the 

1  Baxter  and  Wilson:  Proc.  Anter.  Acad.,  43,  365  (1907);  Jour.  Amer.  Chem.  Soc,  30, 187; 
Zeit.  anorg.  Chem.,  57, 174.     (See  page  67.) 

2  Baxter:  Proc.  Amer.  Acad.,  42,  201  (1906) ;  Jour.  Amer.  Chem.  Soc,  28, 1322;  Zeit.  anorg. 
Chem.,  50,  389.    (See  page  55.) 


134 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


iodide.  Kothner  and  Aeuer  ^  and  Baxter  have  shown  that  silver  iodide  formed  in 
ammoniacal  solution  is  freer  from  occluded  matter  than  when  formed  in  acid 
solution.  This  experiment  was  a  failure,  for  silver  was  reduced  to  the  metallic 
state  owing  to  the  slight  excess  of  hydrazine. 

The  question  of  adsorption  of  silver  nitrate  by  the  precipitate  was  further 
tested  in  several  instances  by  evaporating  the  wash-waters  of  the  silver  iodide 
separately  from  the  filtrate.  In  every  such  case  only  a  mere  trace  of  silver  could 
be  detected,  even  when  the  precipitate  was  allowed  to  stand  in  contact  with 
the  wash- waters  for  several  hours. 

No  experiments  were  performed  to  test  for  complete  reduction  of  the  iodic 
acid  to  hydriodic  acid,  since  evidence  upon  this  point  is  already  available. 
Sammet  ^  has  recently  determined  the  equilibrium  constant  for  the  reaction  of 
iodides  upon  iodates  in  acid  solution  to  be  as  follows: 

(H+)«.(I03-).(I-)^  _ 


2.8 


IO~ 


i.  e.,  lOs"  = 


(12)^ 


(H+)«.(I-)^ 


.2 


10" 


Since  after  reduction  by  the  hydrazine  the  concentration  of  the  free  iodine  ap- 
proximates zero,  and  since  the  concentrations  of  the  hydrogen  and  iodide  ions 
are  fairly  large,  and  since  the  constant  itself  is  extremely  small,  it  is  evident 
that  the  concentration  of  residual  iodate  must  have  been  vanishingly  small. 

A  few  analyses  which  met  with  known  accidents  are  omitted  from  the  fol- 
lowing table  : 

The  Ratio  of  Silver  to  Iodine  Pentoxide. 
Series  II.    By  G.  P.  Baxter. 


No.  of 
analy- 

Sample 
of 

Sample 
of  Ag. 

Corrected 
weight  of  IjOb 

Weight 
of  Ag 

Weight  of 
Agl  from 

Corrected 
weight  of  Ag 

Ratio 
2Ag:l205. 

sis. 

I2O5. 

m  vacuum. 

m  vacuum. 

filtrate. 

m  vacuum. 

gm. 

gm. 

gm. 

gm. 

8 

II 

B 

12.09036 

7-81397 

0.00167 

7.81320 

0.646234 

9 

II 

B4-C 

6.29744 

4.07015 

0.00127 

4.06957 

0.646226 

10 

II 

A 

10.89880 

7.04362 

0.00092  ^ 

7.04309 

0.646226 

II 

II 

D 

9-33895 

6.03554 

0.00106 

6.03505 

0.646222 

12 

II 

A 

10.15370 

6.56194 

0.0005s 

6.56169 

0.646236 

13 

III 

D 

11.00453 

7.I1201 

0.00130 

7.III41 

0.646226 

14 

III 

A 

7.01649 

4-53456 

0.00055 

4.53431 

0.646236 

IS 

III 

D 

9-33573 

6.03362 

0.00125 

6.03304 

0.646231 

16 

III 

D 

8.72163 

5-63666 

0.00103 

5.63619 

0.646231 

17 

III 

D 

9-01524 

5.82603 

0.00025 

5.82591 

0.646229 

i 

Average  . 

.  0.646230 

1  Loc.  cit. 

^  Zeit.  physik.  Chevt.,  53,  640  (1905). 

»  AgBr. 


A  REVISION  OF   THE   ATOMIC  WEIGHTS   OF  IODINE   AND   SILVER.  I35 

DISCUSSION   OF   RESULTS. 

The  average  of  this  series  is  higher  than  that  of  Series  I  by  less  than  one  one- 
thousandth  of  a  per  cent,  the  atomic  weight  of  silver  calculated  from  the  ratio 
of  silver  to  pentoxide  being  107.850.  While  it  is  true  that  impurity  in  the  iodic 
acid  not  containing  halogens  would  tend  to  lower  the  observed  atomic  weight 
of  silver,  the  close  agreement  of  the  two  series  carried  out  with  material  of 
fairly  diverse  nature  practically  eliminates  impurity  in  the  iodic  acid  or  silver 
as  the  cause  of  the  low  resulting  value  for  the  atomic  weight  of  silver.  Sample 
I  of  iodic  acid  was  crystallized  only  three  times  from  aqueous  solution,  while 
Samples  II  and  III  were  both  crystallized  at  least  ten  times.  It  is  improbable 
that  any  impurity  could  have  passed  through  the  additional  crystallizations 
without  appreciable  diminution  in  quantity.  It  has  already  been  shown  that 
mineral  impurities  were  surely  absent  even  before  the  crystallization  of  the 
acid.  Nitric  acid  could  hardly  have  survived  the  prolonged  heating  at  240° 
even  if  it  had  not  been  completely  removed  by  the  many  crystallizations  of  the 
iodic  acid. 

With  regard  to  impurities  containing  halogens  other  than  iodine,  it  may  be 
pointed  out  that  Samples  I  and  III  were  prepared  from  iodine  and  nitric  acid 
which  had  been  very  thoroughly  freed  from  chlorine  and  bromine,  while  even  in 
the  case  of  Sample  II  only  traces  of  chlorine  could  have  been  present.  Aside 
from  these  facts  it  is  decidedly  improbable  that  an  oxygen  acid  of  either  chlorine 
or  bromine  could  have  been  formed  and  then  have  accompanied  the  iodic  acid 
during  its  purification,  for  during  the  heating  of  the  acid  both  before  and  after 
crystallization  such  impurities  would  have  been  either  volatilized  or  destroyed. 
Impurity  of  chlorine  would  probably  tend  to  lower  the  observed  atomic  weight 
of  silver  on  account  of  the  relatively  high  solubiHty  of  silver  chloride.  For 
the  same  reason  a  trace  of  chlorine  in  either  the  hydrazine  or  the  nitric  acid  em- 
ployed in  the  analysis  would  have  had  no  injurious  effect.  Impurity  of  bromine 
would  produce  the  reverse  effect  if  present  in  the  form  of  a  compound  analogous 
to  iodine  pentoxide. 

The  presence  of  a  halogen  of  higher  atomic  weight  than  iodine,  forming  an 
insoluble  silver  salt,  if  present  as  pentoxide  would  lower  the  observed  atomic 
weight  of  silver.  The  existence  of  such  an  element  is  purely  hypothetical, 
however,  and  what  evidence  exists  is  contrary  to  such  an  hypothesis.  One  of 
us  has  recently  searched  for  such  an  element  in  vain.^ 

Two  other  possible  contingencies  must  be  considered,  the  presence  of  either 
free  iodine  or  oxides  of  iodine  higher  than  the  pentoxide.  Free  iodine  might 
result  from  reduction  of  the  pentoxide  during  the  heating.  Such  reduction  or 
decomposition  actually  does  take  place  to  an  extremely  slight  extent  when  the 

^  Baxter:  Proc.  Amer.  Acad.,  40,  422  (1904);  Jour.  Amer.  Chem.  Soc,  26,  1580;  Zeit. 
anorg.  Chem.,  43, 17.     (See  page  93.) 


136  RESEARCHES   UPON  ATOMIC  WEIGHTS. 

pentoxide  is  heated  to  240°,  for  traces  of  free  iodine  can  be  detected  in  air  that 
has  been  passed  over  iodine  pentoxide  at  that  temperature.  That  no  appreciable 
quantity  of  iodine  could  remain  in  the  pentoxide  was  shown  by  the  fact  that  the 
solutions  of  the  pentoxide  were  always  absolutely  colorless  even  when  concen- 
trated. Furthermore  it  was  found  by  experiment  that  a  mere  trace  of  iodine 
could  be  detected  by  its  color  in  such  a  solution.  About  0.0 1  per  cent  of  iodine 
in  the  pentoxide  would  be  necessary  to  raise  the  observed  atomic  weight  of 
silver  by  o.oi  unit. 

Iodine  heptoxide  might  result  from  either  the  presence  of  periodic  acid  in  the 
iodic  acid  or  from  auto-oxidation  of  the  pentoxide  during  the  heating.  Both 
possibilities  are  wanting  in  plausibility,  for  it  is  not  at  all  probable  in  the  light 
of  the  known  instabiUty  of  the  heptoxide  that  the  latter  substance  could  have 
withstood  the  high  temperature  of  heating.  0.025  per  cent  of  heptoxide  in  the 
iodic  acid  would  be  necessary  to  lower  the  observed  atomic  weight  of  silver  by 
0.01  unit. 

It  is  intended  to  pursue  farther  the  study  of  iodic  acid  in  this  laboratory, 
by  the  preparation  of  this  substance  by  other  methods  than  the  action  of  nitric 
acid  on  iodine.  Furthermore,  since  a  very  exact  knowledge  of  the  ratio  of  the 
atomic  weights  of  silver  and  iodine  is  necessary  for  the  computation  of  the  pro- 
portion of  iodine  in  the  pentoxide,  it  is  intended  to  investigate  farther  the  com- 
bining proportions  of  silver  and  iodine,  especially  by  a  method  as  nearly  as 
possible  like  that  employed  in  this  research. 

The  results  of  this  research  may  be  briefly  summed  up  as  follows; 

1.  The  preparation  of  pure  iodic  acid  is  described. 

2.  The  existence  of  the  compoimd  I2O5.HIO3  is  confirmed. 

3.  It  is  shown  that,  while  iodic  acid  may  be  almost  completely  converted 
to  pentoxide  by  heating  at  240°,  a  small  proportion  of  water  remains,  which  is 
constant  for  definite  conditions  of  heating. 

4.  It  is  shown  that  silver  iodide  occludes  silver  sulphate  and  that  sulphur 
dioxide  may  not  be  used  as  a  reducing  agent  if  the  iodine  is  to  be  precipitated 
by  means  of  silver. 

5.  Hydrazine  salts  are  found  to  be  suitable  reducing  agents. 

6.  The  specific  gravity  of  iodine  pentoxide  at  25°  referred  to  water  at  4°  is 
found  to  be  4.80. 

7.  It  is  shown  that  iodine  pentoxide  does  not  adsorb  appreciable  amounts 
of  air. 

8.  The  ratio  of  silver  to  iodine  pentoxide  is  found  to  be  0.646230. 

9.  Upon  this  basis,  if  oxygen  is  assumed  to  be  16.000,  and  if  the  ratio  of  silver 
to  iodine  is  assumed  to  be  0.849943,  the  atomic  weight  of  silver  is  107.850  and 
that  of  iodine  is  126.891. 


IX. 

A   REVISION   OF   THE   ATOMIC   WEIGHT   OF 

CHROMIUM. 

THE  ANALYSIS   OF  SILVER   CHROMATE. 


By  Gregory  Paul  Baxter,  Edward   Mueller,  and  Murray  Arnold  Hines. 


Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  44,  401  (1909). 
Journal  of  the  American  Chemical  Society,  31,  529  (1909). 
Zeitschrift  fiir  anorganische  Chemie,  62,  313  (1909). 
Chemical  News,  100,  180,  189,  199  (1909). 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF 
CHROMIUM. 

THE   ANALYSIS   OF  SILVER  CHROMATE. 


INTRODUCTION. 


The  following  table  ^  gives  the  results  of  investigations  upon  the  atomic 
weight  of  chromium  from  the  time  of  BerzeUus,  recalculated  with  the  use  of 
recent  atomic  weight  ratios  upon  the  basis  of  silver  (107.88)  and  oxygen 
(16.000).^ 


Date. 

Investigator. 

Reference. 

Ratio  determined. 

Result. 

1818 

Berzelius  . 

Pogg.  Ann.  8,  22 

Pb(N03)2:  PbCrO^ 

55-95 

1844 

Peligot  .    . 

Ann.  Chim.  Phys.  (3),  12, 

CrClg:  2AgCl 

52.33 

530 

2CrCl2:  CraOa 
4AgCl:Cr203 

51.58 
51.61 

1846 

Berzelixis  . 

Berzelius'  Jahresb.,  25,  46 

BaCr04  BaS04 

54.5 

1846 

Berlin    .   . 

J.  prakt.  Chem.,  37,  509; 

Ag2Cr04:  2AgCl 

52.65 

38,  149 

2Ag2Cr04 :  CfaOs 
Cr203:4AgCl 
Ag2Cr207:  2AgCl 
Ag2Cr207:Cr208 

52.41 
52.46 
52.11 
52.34 

1848 

Moberg .    . 

Ibid.,  43,  114 

Cr2(S04)8:  CrjOa 

(NH4)2Cr2  (804)4  24HsO:  CrjOs 

53.42 
53.46 

1850 

Lefort    .    . 

Ibid.,  51,  261 

BaCr04:  BaS04 

53.04 

1853 

Wilden  stein 

Ibid.,  59.  27 

BaCh:  BaCr04 

53.56 

1855 

Kessler  .    . 

Pogg.  Ann.,  95,  208 

K2Cr207:  KCIO3 

52.23 

1861 

Kessler  .   . 

Ibid.,  113,  137 

K2Cr207:  KCIO3 
2K2Cr207:3As208 

52.32 
51.92 

1861 

Siewert  .   . 

Zeit.  gesammte  Natur- 

CrCl3:3AgCl 

52.05 

wis.,  17.  530 

Ag2Cr207:  2AgCl 
Cr203:  2AgCl 
Cr203:Ag2Cr207 

52.14 
52.04 
52.05 

1884 

Baubigny  . 

Compt.  Rend.,  98,  146 

Cr2(S04)2:  Cr203 

52.13 

1889 

Rawson     . 

J.  Chem.  Soc,  55.  213 

(NH4)2Cr207:Cr20j 

52.09 

1890 

Meineke    . 

Liebig's  Ann.,  261,  339 

(NH4)2Cr207:Cr20s 
2Ag2Cr04:  Cr203 
Ag2Cr04:  2AgCl 
4AgCl:  CrjOs 
2Ag2Cr04  4NH3:Cr20s 
Ag2Cr04  4NH3:2AgCl 
4AgCl:  Cr203 
Ag2Cr04: 3I 
Ag2Cr04  4NH3:3l 
K2Cr207:  KHIO3 
(NH4)2Cr207:KHIOj 

52.11 
52.10 
52.03 
52.14 
52.27 
51.62 
52.14 
52.41 
52.05 
52.14 
52.13 

*  Clarke,"  A  Recalculation  of  the  Atomic  Weights,"  Smith.  Misc.  Coll.,  19 10. 

«  The  following  atomic  weights  are  used  in  the  calculation  of  the  older  values:  Ag  = 
107.88;  CI  =  35.46;  Pb  =  207.09;  N  =  14.01;  Ba  =  137.37;  S  =  32.07;  H  =  1.008;  K 
=  39.10;  As  =  74.96;  I  =  126.92.  The  values  of  Rawson  and  Meineke  are  reduced  to  the 
vacuum  standard;  the  others  are  not  so  corrected. 

139 


140 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


It  has  been  repeatedly  shown,  especially  in  this  laboratory,  that  most  of  the 
earlier  work  upon  atomic  weights  has  been  vitiated  by  neglect  of  certain  fun- 
damental precautions.  The  incomplete  drying  of  solids  has  been  responsible 
for  many  of  the  discrepancies  and  errors  which  exist.  Neglect  of  the  solubility 
of  precipitates,  together  with  the  use  of  too  concentrated  solutions  during 
precipitation,  so  that  perceptible  inclusion  and  occlusion  took  place,  undoubt- 
edly have  influenced  many  gravimetric  processes.  Volumetric  processes  have 
been  affected  by  inaccurately  prepared  standard  solutions,  as  well  as  the  diffi- 
culty inherent  in  measuring  exactly  large  volumes  of  solution. 

In  discussing  in  detail  the  appHcations  of  the  above  causes  of  constant  error 
to  the  individual  investigations,  at  the  best  it  is  possible  merely  to  indicate 
the  nature  of  the  difficulties;  as  a  rule  it  is  impossible  to  estimate  the  magnitude 
of  the  error  without  repetition  of  the  experimental  work.  Hence  in  this  paper 
attention  is  called  only  to  points  in  the  earlier  work  which  have  been  experi- 
mentally investigated.  The  uncertainty  in  most  of  the  previous  determina- 
tions is  emphasized  by  the  lack  of  agreement  in  the  individual  analyses  in  each 
series,  as  well  as  in  the  different  series. 

The  choice  of  method  for  this  investigation  was  influenced  by  several  con- 
siderations. In  the  first  place,  the  substance  to  be  analyzed  must  be  definite  in 
composition  and  capable  of  being  either  fused  or  heated  to  a  high  temperature 
in  order  to  insure  the  elimination  of  moisture.  In  the  second  place,  in  view  of 
the  fact  that  chromium  is  hard  to  handle  satisfactorily  in  a  quantitative  fashion, 
the  analytical  operation  should  involve  the  determination  of  some  other  ele- 
ment. The  halogen  compounds,  which  have  been  employed  very  successfully 
many  times,  especially  in  this  laboratory,  for  the  determination  of  the  atomic 
weights  of  metallic  elements,  are  less  suited  for  use  in  the  case  of  chromiimi  on 
account  of  the  difficulty  in  the  complete  precipitation  of  the  halogens  by  means 
of  silver  nitrate. 

All  things  considered,  the  chromates  of  silver  seemed  to  offer  the  most  prom- 
ising possibilities  on  account  of  the  ease  with  which  their  silver  content 
may  be  determined.  It  is  true,  in  order  to  determine  the  ratio  of  the  atomic 
weight  of  chromium  to  that  of  either  silver  or  oxygen,  this  method  neces- 
sitates a  knowledge  of  the  exact  ratio  of  the  atomic  weights  of  silver  and  oxy- 
gen, knowledge  which  is  still  slightly  uncertain.  The  per  cent  of  silver  in  the 
compound  being  known,  however,  analytical  data  may  be  used  at  any  subse- 
quent time  for  the  calculation  of  the  atomic  weight  of  chromixmi.  Furthermore, 
since  the  value  for  the  atomic  v/eight  of  chromium  at  present  accepted  depends 
very  largely  upon  the  analysis  of  silver  chromate,  a  study  of  this  salt  with  the 
application  of  the  most  modern  methods  seemed  to  promise  interesting  results, 
and  therefore  was  first  taken  up.  In  a  following  paper  is  given  a  description  of 
the  analysis  of  silver  dichromate. 


^  THE   ANALYSIS   OF    SILVER   CHROMATE.  I41 

PURIFICATION    OF   MATERIALS. 

The  laboratory  distilled  water  was  twice  redistilled,  once  from  alkaline  per- 
manganate and  once  from  very  dilute  sulphuric  acid.  In  both  distillations 
block  tin  condensers  were  employed,  no  cork  or  rubber  connections  being  nec- 
essary. 

The  preparation  of  pure  neutral  silver  nitrate  for  the  precipitation  of  silver 
chromate  followed  the  lines  laid  down  in  previous  researches  in  this  laboratory. 
A  large  quantity  of  heterogeneous  silver  residues  was  reduced  to  metallic  silver 
by  means  of  sticks  of  pure  zinc  in  slightly  acid  solution.  After  the  silver  had 
been  washed  with  water  imtil  free  from  halogens,  it  was  dissolved  in  nitric  acid, 
and  the  solution  was  filtered.  Silver  chloride  was  precipitated  from  the  diluted 
nitrate  by  means  of  hydrochloric  acid,  and  the  precipitate  of  silver  chloride 
was  thoroughly  washed.  From  this  silver  chloride,  metallic  silver  was  again 
obtained  by  reduction  with  cane  sugar  in  strongly  alkaline  solution.  After 
being  washed  until  free  from  chloride,  the  metal  was  again  dissolved  in  nitric 
acid  in  a  Jena  glass  flask.  By  reduction  with  ammonic  formate  (prepared  from 
redistilled  formic  acid  and  redistilled  ammonia),  the  silver  was  once  more  ob- 
tained in  the  metallic  state.  The  crystals  were  then  dissolved  in  the  purest 
nitric  acid,  and  the  nitrate,  after  concentration  of  the  solution,  was  four  times 
recrystallized  from  the  purest  water  in  platinum  until  free  from  acid.  In  this 
crystallization,  and  in  all  others,  centrifugal  drainage  in  a  machine  employing 
platinmn  fimnels  as  baskets  ^  was  always  used,  in  order  to  free  the  crystals 
entirely  from  any  adhering  mother-liquor,  the  mother-Hquors  all  being  rejected. 

Hydrochloric  acid  was  prepared  by  distilling  the  commercial  chemically 
pure  acid,  after  dilution  with  an  equal  volume  of  water. 

The  methods  for  obtaining  piure  bromine  have  been  recently  tested  by  one  of 
us,2  and  the  processes  found  suitable  for  the  purpose  were  employed  here.  A 
considerable  quantity  of  hydrobromic  acid  was  prepared  by  passing  a  current 
of  pure  hydrogen  sulphide  through  a  layer  of  bromine  covered  with  water.  The 
hydrogen  sulphide  was  generated  by  the  action  of  dilute  sulphuric  acid  on 
ferrous  sulphide,  and  was  thoroughly  scrubbed  in  gas-washing  bottles  and 
towers  containing  water.  After  the  precipitated  mixture  of  sulphur  bromide 
and  sulphm:  had  been  removed  by  decantation  and  filtration,  the  acid  was 
boiled,  with  the  occasional  addition  of  small  portions  of  recrystallized  potas- 
sium permanganate.  This  was  done  to  eliminate  any  iodine  which  might  have 
been  present. 

The  hydrobromic  acid  was  then  heated  with  the  calculated  quantity  of  re- 
crystallized potassium  permanganate,  the  bromine  being  condensed  in  a  Jena 
flask  cooled  with  running  water.  In  this  way  three-eighths  of  the  bromine  re- 
mained behind  as  potassiimi  and  manganous  bromides,  the  remaining  five- 

^  Richards:  Jour.  Amer.  Chem.  Soc,  27,  no  (1905). 

2  Baxter:  Proc.  Amer.  Acad.,  42,  204  (1906);  Jour.  Amer.  Chem.  Soc,  28, 1327;  Zeit.  anorg. 
Chem.,  so,  394.    (See  page  54.) 


142  RESEARCHES   UPON   ATOMIC  WEIGHTS. 

eighths  being  distilled  from  the  solution  of  these  bromides.  The  greater  part  of 
the  chlorine  was  undoubtedly  eliminated  by  this  operation,  since  the  original 
bromine  was  fairly  pure.  In  order  to  be  on  the  safe  side,  however,  the  bromine 
was  again  reduced  to  hydrobromic  acid,  and  this  in  turn  was  changed  to  bromine 
as  above.  From  the  product  the  final  hydrobromic  acid  was  prepared  with 
hydrogen  sulphide.  After  filtration  and  distillation,  it  was  preserved  in  Jena 
glass. 

Chromic  acid  was  prepared  from  Merck's  "Highest  Purity  Chromic  Acid." 
The  material  was  dissolved  in  pure  water,  and  the  solution  was  filtered  through 
a  Gooch-Munroe-Neubauer  crucible  with  a  mat  of  platimun  sponge,  a  quan- 
tity of  sandy  material  being  thus  separated.  The  solution  was  then  evap- 
orated to  saturation  and  three  times  systematically  recrystallized  in  platinum 
dishes  with  centrifugal  draining,  each  mother-liquor  being  used  for  the  crystalli- 
zation of  three  crops  of  crystals  on  account  of  the  small  temperature  coefficient 
of  solubility  of  chromic  acid.  The  mother-liquors  from  the  first  crystallization, 
on  testing  in  the  nephelometer,  indicated  only  traces  of  sulphates  and  halogens. 

Some  of  the  purest  commercial  potassium  chromate,  after  solution  in 
water,  was  filtered  through  a  Gooch-Munroe-Neubauer  crucible.  It  was  then 
four  times  crystallized  in  platinimi,  each  crop  of  crystals  being  centrifugally 
drained. 

PREPARATION    OF   SILVER   CHROMATE. 

The  point  in  the  investigation  requiring  the  most  attention  was  the  prepa- 
ration of  normal  silver  chromate  free  from  both  basic  and  acid  salts.  Since 
the  salt  can  not  be  crystallized,  owing  to  its  sKght  solubility  in  water,  it  is  neces- 
sary so  to  regulate  the  conditions  during  precipitation  that  neither  acid  nor 
basic  salts  can  separate  as  distinct  solid  phases.  Even  then  the  occlusion  of 
traces  of  either  basic  or  acid  salts  is  still  possible,  and  it  is  necessary  to  form  the 
salt  under  a  fairly  wide  range  of  conditions  in  order  to  show  constancy  of  com- 
position. 

Fortunately  data  are  available  which  indicate  the  conditions  under  which 
silver  dichromate  or  hydrochromate  can  exist.  Sherrill  ^  has  recently  shown 
that  silver  chromate  changes  into  silver  dichromate  rapidly  under  a  saturated 
solution  in  nitric  acid  more  concentrated  than  0.075  normal,  while  silver  di- 
chromate changes  into  silver  chromate  under  a  saturated  solution  in  nitric  acid 
less  concentrated  than  0.06  normal.  Some  time  before,  Kriiss  ^  had  shown  that 
silver  dichromate  is  converted  into  silver  chromate  by  contact  with  water. 

In  the  light  of  these  facts  it  is  obvious  that  the  solutions  of  the  soluble  chro- 
mates  can  safely  be  employed  for  the  precipitation  of  silver  chromate  without 
the  least  danger  of  the  precipitation  of  silver  dichromate,  and  even  that  the 
presence  of  a  slight  amount  of  free  acid  could  do  no  harm. 

*  Jour.  Amer.  Chent.  Soc,  29,  1673  (1907). 

*  Ber.  d.  d.  Ghent.  Gesell.,  22,  2050  (1889). 


THE   ANALYSIS   OF   SILVER  CHROMATE.  143 

Owing  to  the  weak  nature  of  the  second  hydrogen  of  chromic  acid,  the  first 
hydrogen  dissociating  to  the  same  extent  as  that  of  hydrochloric  acid,^  but  the 
second  hydrogen  having  the  constant  6.0  X  10"'  at  18°,^  appreciable  hydrolysis 
of  solutions  of  its  salts  takes  place,  to  a  greater  extent  the  weaker  the  base  with 
which  the  chromic  acid  is  combined.  Sherrill  has  found,  for  instance,  that  am- 
monium chromate  in  0.05  molal  solution  is  2.7  per  cent  hydrolyzed.  The  basic- 
ity of  the  solutions,  on  the  other  hand,  will  be  greater  the  stronger  the  base.  In 
order  to  determine  whether  this  hydrolysis  is  sufficient  to  produce  precipita- 
tion or  occlusion  of  basic  chromates,  precipitates  of  silver  chromate  were 
formed  by  means  of  solutions  of  both  ammonium  and  potassium  chromates. 
The  comparison  of  precipitates  formed  in  this  way  will  show  whether  the  pres- 
ence of  basic  salts  is  to  be  feared. 

Sample  I.  —  Ammonic  chromate  was  prepared  by  adding  to  a  solution  of  the 
pure  chromic  acid  a  slight  deficiency  of  the  purest  freshly  distilled  ammonia. 
The  solution  was  diluted  until  about  tenth  normal,  and  was  slowly  poured  with 
constant  shaking  into  a  solution  of  an  equivalent  quantity  of  silver  nitrate  of 
about  the  same  concentration.  The  dark  red  precipitate  of  silver  chromate  was 
washed  six  times  by  decantation  with  large  portions  of  water,  centrifugally 
drained  to  remove  as  much  water  as  possible  and  dried  at  gradually  increasing 
temperatures  in  an  electric  oven,  finally  at  160°  for  a  long  time.  The  dried 
lumps  were  then  gently  ground  to  a  fine  powder  in  an  agate  mortar  in  order  to 
facilitate  further  drying  as  well  as  to  insure  homogeneity.  During  the  addition 
of  the  chromate  to  the  silver  solution,  since  the  chromate  solution  was  slightly 
deficient  in  ammbnia,  acid  acciunulated  in  the  silver  nitrate  solution.  Hence 
each  succeeding  portion  of  precipitate  was  formed  under  conditions  of  greater 
acidity,  although  the  concentration  of  acid  in  the  solution  could  never  have 
approached  that  found  by  Sherrill  to  be  necessary  for  the  existence  of  the  silver 
dichromate. 

Sample  II.  —  This  preparation  was  practically  identical  with  Sample  I,  since 
part  of  the  precipitate  obtained  as  above  was  washed  by  decantation  with 
water  eight  times  more,  each  wash-water  being  allowed  to  stand  in  contact 
with  the  precipitate  for  many  hours,  and  the  precipitate  being  shaken  with  the 
wash-water  very  thorbughly  at  intervals,  in  order  to  leach  out  any  acciden- 
tally inclosed  or  adsorbed  soluble  salts.  The  prolonged  extra  washing  evidently 
was  unnecessary,  since  the  results  are  practically  the  same  as  those  obtained 
with  Sample  I. 

Sample  III.  —  This  sample  was  prepared  from  the  four  times  recrystallized 
potassium  chromate.  A  quantity  of  this  material  in  about  tenth  normal  solu- 
tion was  precipitated  with  an  equivalent  amount  of  silver  nitrate,  equally 
dilute.  The  precipitation  took  place  in  Jena  glass,  the  silver  solution  being 
slowly  poured  into  the  chromate,  in  order  to  accentuate  the  effect  of  the  hy- 
drolysis if  possible.    It  will  be  recalled  that  in  the  case  of  Samples  I  and  II  pre- 

1  Walden:  Zeit.  physikal.  Chem.,  2,  49  (1888).  2  Sherrill,  Loc.  cit. 


144  RESEARCHES   UPON   ATOMIC  WEIGHTS. 

pared  with  the  ammonium  salt,  the  chromate  was  added  to  the  silver  solution. 
The  precipitate  was  then  transferred  to  platinum  and  washed  seven  times  with 
the  purest  water,  the  chromate  being  thoroughly  agitated  with  each  washing. 
After  the  removal  of  the  greater  part  of  the  adhering  water  by  centrifugal 
settling,  this  sample  was  dried  in  a  preliminary  fashion  at  150°  and  was 
pulverized  in  an  agate  mortar,  as  in  the  case  of  Sample  I  and  II.  The 
salt  was  soft  and  crystalline,  and  greenish  black  in  color. 

Sample  IV.  —  A  fourth  sample  also  was  prepared  from  recrystallized  potas- 
sium chromate,  which  in  turn  was  made  from  recrystallized  chromic  acid.  In 
the  first  place,  potassium  hydroxide  was  prepared  by  the  electrolysis  of  three 
times  recrystallized  potassium  oxalate,  with  the  use  of  a  mercury  cathode  and 
decomposition  of  the  amalgam  with  pure  water  in  a  platinimi  dish,  as  in  the 
preparation  of  potassium  hydroxide  in  an  investigation  upon  the  atomic  weight 
of  potassium.^  The  solution  of  the  pure  hydroxide  was  added  to  a  solution  of 
three  times  recrystallized  chromic  acid,  contained  in  a  platinum  dish,  until  the 
normal  chromate  had  been  formed  as  indicated  by  the  yellow  color.  From  this 
solution,  by  three  systematic  crystallizations,  potassium  chromate  was  separated. 

The  silver  chromate  was  prepared  from  this  material  and  the  purest  silver 
nitrate  by  slowly  adding  a  six-hundredths  normal  solution  of  the  chromate  to  a 
silver  nitrate  solution  of  equivalent  concentration,  this  procedure  being  the  re- 
verse of  that  used  in  the  preparation  of  Sample  III.  By  this  method  of  precipi- 
tation the  solution  is  maintained  as  nearly  neutral  as  it  is  possible  to  keep  it. 
The  dark  brownish-red  precipitate  was  allowed  to  settle  in  the  flask  in  which 
precipitation  took  place.  Then,  the  supernatant  solution  having  been  decanted, 
the  silver  chromate  was  transferred  to  a  platinum  dish  and  washed  very  thor- 
oughly with  water.  After  being  freed  from  water  by  centrifugal  settling,  the  sil- 
ver chromate  was  dried  at  about  160°  in  an  electric  oven,  and  powdered  in  an 
agate  mortar. 

Since  in  the  case  of  Sample  III  the  silver  nitrate  was  added  to  the  chromate, 
while  in  preparing  Sample  IV  precipitation  took  place  in  the  reverse  fashion,  a 
comparison  of  the  two  samples  would  not  only  throw  light  upon  the  effect  of 
hydrolysis,  but  also  show  whether  the  occlusion  of  potassium  chromate  or 
silver  nitrate  was  to  be  feared. 

DRYING   OF   SILVER    CHROMATE. 

The  fact  that  salts  dried  by  prolonged  heating  at  100°,  or  at  even  higher 
temperatures,  usually  contain  appreciable  amounts  of  moisture,  owing  to  in- 
cluded mother-liquor,  is  a  point  which  has  been  overlooked  by  most  earlier  in- 
vestigators,^  and  the  oversight  throws  doubt  on  much  otherwise  very  careful 
work.    In  exact  work  the  residual  water  must  either  be  corrected  for  or  entirely 

^  Richards  and  Mueller:  Pub.  Car.  Inst.,  69,  33  (1907);  Jour.  Amer.  Chetn.  Soc,  29,  645; 
Zeii.  anorg.  Chem.,  53,  431. 

*  Richards:  Proc.  Amer.  Phil.  Soc,  42,  28  (1903). 


THE  ANALYSIS   OF   SILVER  CHROMATE.  145 

avoided.  The  simplest  fashion  of  drying  a  substance  perfectly  is  to  fuse  it  in  a 
current  of  dry  gas.  In  the  case  of  the  silver  chromate,  however,  this  is  not 
practicable,  for  even  at  300°  incipient  decomposition  sets  in.  Upon  attempt- 
ing to  dissolve  in  nitric  acid  samples  dried  in  air  at  that  temperature,  a  slight 
insoluble  residue  was  always  obtained,  while  heating  in  a  current  of  oxygen 
gave  no  better  results.  Since  the  moisture  can  not  be  entirely  expelled  from 
silver  chromate  by  heating  at  a  moderate  temperature,  it  must  be  determined 
by  the  analysis  of  separate  portions  of  the  substance  which  have  been  treated 
in  some  definite  fashion. 

Experiments  showed  that  at  temperatures  below  225°  the  salt  was  not 
appreciably  changed,  hence  this  temperature  was  chosen  as  a  suitable  one  at 
which  to  heat  the  salt  preparatory  to  analysis.  The  silver  chromate  was  there- 
fore always  heated  in  a  current  of  pure  dry  air  for  2  hours  at  225°,  in  order  to 
obtain  the  separate  portions  in  as  nearly  as  possible  the  same  condition. 

The  drying  apparatus  was  constructed  entirely  of  glass,  rubber  connections 
being  especially  avoided.  A  current  of  air  was  passed  first  over  red-hot  copper 
oxide  to  destroy  organic  matter,  then  through  successive  EmmerHng  washing 
towers.  In  the  first  were  beads  drenched  with  silver  nitrate  solution,  in  the 
second  with  a  strong  solution  of  potassic  hydroxide  containing  much  potassic 
manganate,  and  in  the  last  three  with  concentrated  sulphuric  acid.  The  already 
very  dry  air  was  then  passed  through  a  long  tube  containing  resublimed  phos- 
phoric anhydride  spread  over  a  large  surface  of  glass  beads  and  ignited  asbestos. 
From  the  drying  apparatus  the  air  passed  into  the  tube  in  which  the  boat  con- 
taining the  silver  chromate  was  placed. 

DETERMINATION    OF   SILVER   IN   SILVER   CHROMATE. 

During  the  drying  of  the  silver  chromate  it  was  contained  in  a  platimmi  boat 
which  had  been  weighed,  in  a  weighing-bottle,  by  substitution  for  a  similar 
bottle  which  with  its  contents  displaced  the  same  amount  of  air  as  the  bottle 
with  the  boat.  The  boat  was  placed  in  a  hard-glass  tube  connected  by  a  care- 
fully ground  joint  with  the  bottling  apparatus.^  The  tube  was  heated  by  means 
of  the  solid  aluminum  oven  described  on  page  78.  After  2  hours'  heating  at 
225°  the  boat  was  transferred  to  the  weighing-bottle  and  was  allowed  to  stand 
in  a  desiccator  near  the  balance  for  several  hours  before  being  weighed. 

Next,  the  weighed  quantity  of  silver  chromate  was  transferred  to  a  3-liter 
glass  stoppered  Jena  flask  with  a  carefully  ground  stopper,  and,  after  the  boat 
and  bottle  had  been  cleaned  with  hot  dilute  nitric  acid  and  water,  the  rinsings 
were  poured  into  the  flask  and  the  silver  chromate  dissolved  by  the  appUca- 
tion  of  gentle  heat.  If  the  salt  had  not  been  heated  above  225°,  the  solution 
was  absolutely  clear.  Specimens  heated  above  this  temperature  always  showed 
more  or  less  turbidity. 

*  See  page  8. 


146  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

The  chromate  was  next  reduced  to  the  chromic  state  by  the  addition  of  a  very 
slight  excess  of  sulphur  dioxide  which  had  been  freshly  distilled  into  pure  water. 
The  slight  excess  of  sulphurous  acid  was  soon  oxidized  under  the  combined 
influence  of  heat  and  nitric  acid.  In  analyses  1,2,  3, 12, 13,  and  14  the  reduc- 
tion was  effected  by  means  of  recrystallized  hydrazine  sulphate,  in  order  to 
avoid  to  a  large  extent  the  presence  of  sulphuric  acid,  for  Richards  and  Jones  ^ 
found  that  silver  chloride  occludes  silver  sulphate  very  tenaciously.  The  two 
methods  of  reduction,  however,  gave  essentially  identical  results. 

Since  in  the  reduction  of  the  chromate  by  hydrazine,  nitrogen  gas  is  evolved, 
the  flask  in  which  the  reduction  took  place  was  protected  from  loss  by  spatter- 
ing by  means  of  a  long  column  of  bulbs  fitting  loosely  into  the  neck  of  the  flask. 
The  solution  of  hydrazine  sulphate  was  added  through  a  funnel  with  a  long  fine 
stem  which  extended  through  the  column  of  bulbs  nearly  to  the  bottom  of  the 
flask.  After  the  addition  of  the  hydrazine,  the  reaction  was  allowed  to  continue 
slowly,  with  occasional  shaking,  and  was  completed  by  heating  the  solution 
upon  a  steam  bath  for  a  short  time.  In  the  presence  of  acid  a  dilute  solution  of 
hydrazine  is  without  effect  upon  silver  salts. 

After  the  solution  had  been  allowed  to  cool,  it  was  diluted  to  a  volume  of  1.5 
liters,  and  the  silver  was  precipitated  as  chloride  or  bromide  by  the  addition 
of  a  very  dilute  solution  of  an  excess  of  either  hydrochloric  or  hydrobromic  acid. 
The  flask  with  its  contents  was  shaken  thoroughly  for  a  few  moments  and  was 
then  allowed  to  stand  several  days,  until,  the  silver  bromide  having  settled, 
the  supernatant  solution  was  perfectly  clear. 

Since  the  mother-liquor  of  the  silver  halide  contained  both  nitric  and  hydro- 
bromic acids  in  excess,  the  use  of  a  Gooch-Munroe-Neubauer  crucible  seemed 
to  be  attended  with  danger  on  account  of  solution  of  platinum.  Such  a  possi- 
bility has  already  been  pointed  out,^  and  an  actual  loss  was  found  to  take  place 
in  blank  experiments  carried  out  at  the  beginning  of  this  research.  Accordingly 
the  ordinary  platinum  Gooch  crucible  with  an  asbestos  mat  was  used.  The 
asbestos  had  been  carefully  cleansed  by  ignition,  and  washing  first  with  nitric 
acid  and  then  with  water.  The  crucible  was  prepared  for  weighing  before  and 
after  filtration  of  the  silver  haHde  in  exactly  the  same  way. 

The  silver  haUdes  were  washed  many  times  by  decantation  with  dilute  hy- 
drochloric acid  in  the  case  of  silver  chloride,  and  with  very  dilute  hydrobromic 
acid  in  the  case  of  silver  bromide.  The  precipitate  was  then  transferred  to  the 
weighed  crucible  and  was  dried  in  an  electric  oven  at  170°  for  at  least  16  hours. 

In  order  to  correct  for  the  small  quantity  of  moisture  retained  by  the  silver 
haUdes,  each  precipitate  was  transferred  as  completely  as  possible  to  a  porcelain 
crucible  and  fused.  From  the  loss  of  weight  of  the  portion  of  silver  salt  trans- 
ferred to  the  crucible,  the  amount  of  water  in  the  entire  precipitate  was  cal- 
culated. 


»  Pub.  Car.  Inst.,  69,  73  (1907);  Jour.  Amer.  Chem.  Soc,  29,  831;  Zeit.  anorg.  Chem.,  55,  84. 
»  Morse,  "  Exercises  in  Quantitative  Chemistry,"  p.  203  (1905). 


^  THE  ANALYSIS   OF   SILVER  CHROMATE.  I47 

The  small  quantity  of  asbestos,  together  with  a  trace  of  silver  bromide 
which  escaped  the  crucible,  was  collected  by  passing  the  entire  filtrate  and  wash- 
ings through  a  small  filter.  The  ash  of  this  filter  was  treated  with  nitric  and 
with  hydrochloric  or  hydrobromic  acid,  then  it  was  reheated  and  the  crucible 
was  weighed.  After  correction  for  the  ash  of  the  filter,  the  gain  in  weight  of 
the  crucible  was  added  to  the  weight  of  the  main  mass  of  silver  halide. 

Another  correction  was  necessary.  The  filtrate  contained  dissolved  silver 
salt,  even  though  an  excess  of  halogen  acid  was  used  in  the  precipitation. 
The  larger  part  of  the  dissolved  halide  is  due  to  the  marked  solubility  in  solutions 
of  chromic  salts,  the  amount  dissolved  increasing  with  increasing  concentration 
of  the  chromic  salts.  Berlin  overlooked  this  correction  which  was  afterwards 
pointed  out  by  Siewert.  Meineke  later  determined  experimentally  the  quantity 
of  dissolved  material,  and  also  proposed  the  method  of  separation  which  was 
adopted  in  this  work.  The  entire  filtrate  of  3  to  4  liters  was  evaporated  to  small 
bulk,  nearly  neutralized  with  ammonia,  and  then  the  silver  was  precipitated 
from  a  hot  solution  as  sulphide.  The  precipitate  was  collected  upon  a  filter 
paper,  which  was  ignited.  The  residue  was  converted  to  the  nitrate  by  digestion 
with  dilute  nitric  acid,  and  the  solution  was  then  filtered  into  a  graduated  flask, 
in  which  it  was  diluted  to  known  volume.  By  comparison  in  the  nephelometer 
of  this  solution  with  standard  solutions  of  silver  the  quantity  of  silver  in  solu- 
tion was  determined. 

That  all  dissolved  silver  was  recovered  in  this  way  was  shown  by  adding  an 
excess  of  ammonia  to  the  filtrate  of  the  silver  sulphide  in  one  analysis,  the  hy- 
drogen sulphide  having  been  expelled,  and  after  removal  of  the  chromic  hydrox- 
ide by  filtration,  testing  the  acidified  filtrate  for  silver.   None  could  be  detected. 

DETERMINATION    OF   MOISTURE   IN    DRIED   SILVER   CHROMATE. 

The  proportion  of  moisture  in  the  silver  chromate  was  foimd  by  fusing 
weighed  quantities  of  the  salt  in  a  current  of  pure  dry  air  and  collecting  the 
water  vapor  produced  in  a  weighed  phosphorus  pentoxide  tube.  During  the 
fusion  of  the  salt  oxgyen  is  evolved,  but  since  the  fusing  point  is  low,  there  is  on 
danger  of  volatilization  of  either  silver  or  chromium  compounds. 

In  order  to  avoid  the  necessity  of  removing  the  fused  silver  chromate  from  a 
platinum  boat,  boats  of  copper  foil  which  had  been  cleaned  and  ignited  were 
employed. 

It  was  desirable  to  determine  not  only  whether  the  proportion  of  water  could 
be  made  constant  at  any  one  temperature,  but  also  how  much  the  proportion 
of  water  is  affected  by  variations  in  temperature.  Experiments  were  therefore 
carried  out  with  silver  chromate  which  had  been  dried  for  2  hours  at  200°,  225°, 
and  300°,  in  dry  air  which  had  been  purified  as  previously  described. 

After  the  salt  had  been  dried,  a  carefully  weighed  U-tube  containing  resub- 
limed  phosphorus  pentoxide  was  attached  to  the  end  of  the  tube,   This  U-tube 


148 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


was  provided  with  ground  glass  stopcocks  lubricated  with  Ramsay  desiccator 
grease.  The  silver  chromate  was  gradually  heated  until  fusion  took  place,  and 
a  slow  current  of  air  was  allowed  to  pass  through  the  system  for  one  half  hour 
in  order  to  make  certain  that  all  moisture  was  carried  into  the  absorption  tube. 
Finally  the  phosphorus  pentoxide  tube  was  reweighed. 


Temperature 
of  heating. 

Weight  of  sil- 
ver chromate. 

Weight  of 
water. 

Per  cent 
of  water. 

200° 
200° 
200° 

gm. 
4.87 
4-74 
4-43 

gm. 
0.00097 
0.00098 
0.00093 

0.0199 
0.0207 
0.0210 

Average 0.0205 

225° 
225° 

225° 

225° 

225° 

9.01 
10.85 

lO.II 

7-95 
8.23 

0.00136 
0.00188 
0.00125 
0.00105 
O.OOII4 

0.0151 
0.0173 
0.0124 
0.0132 
0.0139 

Average 0.0144 

300° 

3-50 

0.00034 

0.0097 

The  pentoxide  tube  was  weighed  by  substitution  with  the  use  of  a  counter- 
poise of  the  same  size  and  weight.  Before  being  weighed  both  tubes  were  care- 
fully wiped  with  a  damp  cloth  and  were  allowed  to  stand  near  the  balance  case 
for  30  minutes.  Care  was  taken  to  equalize  the  pressure  inside  and  outside  the 
tubes  by  opening  one  stopcock  immediately  before  hanging  on  the  balance. 

In  order  to  test  the  efficiency  of  the  drying  apparatus,  blank  experiments  were 
carried  out  by  allowing  a  slow  current  of  air  to  pass  through  the  apparatus  into 
the  weighed  pentoxide  tube.  The  variations  in  the  weight  of  the  tube  were 
never  much  larger  than  the  probable  error  in  weighing  the  tube. 

As  is  to  be  expected,  the  water  content  gradually  decreases  with  increasing 
temperature  of  heating.  The  extreme  variation  with  specimens  of  silver 
chromate  which  have  been  heated  at  225°  amounts  to  only  0.005  P^r  cent.  Evi- 
dently the  percentage  of  residual  water  is  as  constant  as  can  be  reasonably 
expected,  and  the  mean  can  safely  be  assumed  to  represent  with  sufficient 
exactness  the  average  proportion  of  water  in  the  salt.  Hence  from  every 
apparent  gram  of  silver  chromate  0.000144  gm.  is  subtracted. 


THE   ANALYSIS   OF   SILVER  CHROMATE. 
SPECIFIC    GRAVITY   OF   SILVER   CHROMATE. 


149 


In  order  to  correct  tte  weight  of  sfilver  chromate  to  a  vacuum  Standard,  a 
knowledge  of  its  specific  gravity  is  necessary.  This  has  already  been  deter- 
mined by  Playfair  and  Joide  ^  and  Schroeder,^  who  obtained  the  values  5.77 
and  5.53  respectively.  On  account  of  the  marked  difference  between  these 
values,  new  determinations  of  the  density  were  made  by  the  displacement  of 
toluol  with  weighed  amounts  of  salt.  The  toluol  was  first  dried  by  stick  soda 
and  was  then  distilled.  Its  specific  gravity  at  25°  referred  to  water  at  4°  was 
found  to  be  0.86156.  Great  pains  were  taken  to  remove  air  from  the  chromate 
when  covered  with  toluol  by  placing  the  pycnometer  in  an  exhausted  desiccator 
before  setting. 

The  Specific  Gravity  of  Silver  Chromate. 


Weight  of  silver 

chromate  in 

vacuum. 

Weight  of  toluol 

displaced  in 

vacuum. 

Density  of 
silver  chromate. 

25° /4°. 

gm. 

5-1584 
3.6012 

gm. 
0.7898 
0.5520 

S.628 
5-621 

Average 

.          S-625 

The  following  vacuum  corrections  were  applied: 


Specific  gravity. 

Vacuum  correction. 

Weights 

Toluol 

Silver  chromate   .... 

Silver  chloride 

Silver  bromide     .... 

8.3 

0.862 

5-625 

S-56 

6.473 

+0.00126 
+0.000069 
+0.000071 
+0.000041 

BALANCE  AND  WEIGHTS. 

All  weighings  were  made  by  substitution  upon  a  nearly  new  short-armed 
Troemner  balance,  easily  sensitive  to  0.02  mg.  with  a  load  of  50  gm. 

The  gold-plated  Sartorius  weights  were  carefully  standardized  by  the  method 
described  by  Richards,^  and  were  used  for  no  other  work. 

^  Mem.  Chem.  Soc,  2,  401  (1845). 

^  Lieb.  Ann.,  173,  72  (1874). 

^  Jour  Amer  Chem.  Soc,  2a,  744  (igcc) 


ISO 


RESEARCHES  UPON  ATOMIC  WEIGHTS. 


Series  I.    2 AgCl :  AgjCrO*. 

Ag 


AgCI 


=  0.7526321 


No.  of 
analy- 
sis. 


Sample 

of 
AgjCr04. 


II 
II 

IV 


Corrected 

weight  of 

AgjCrO«  in 

vacuum. 


gm. 

10.30985 

8.26920 

6.56679 


Weight  of 
AgCl  in 
vacuum. 


8.90835 
7.14327 
5-67324 


Loss  on 
fusion. 


gm. 
0.00063 
0.00063 
0.00039 


Weight  of 
asbestos 


gm. 
0.00117 
O.00211 
0.00136 


Dissolved 

AgCl  from 

filtrate. 


gm. 
0.00019 
0.00017 
0.00023 


Corrected 

weight  of 

AgCl  in 

vacuum. 


8.90908 
7.14492 
5-67444 


Ratio 

2  AgCl: 

AgjCrO,. 


0.864132 
0.864040 
0.8641 1 1 


Average 0.864094 

Per  cent  of  Ag  in  Ag2Cr04 65.0345 


Series  II.    2  AgBr:  Ag2CrO<. 

Ag  . 

AiBr  =  °-5744S3' 


No.  of 
analy- 
sis. 

Sample 

of 
AgjCrO^. 

Corrected 
weight  of 
AgzCrOi 

in 
vacuum. 

Weight  of 

AgBr 

m 

vacuum. 

Loss  on 
fusion. 

Weight  of 
asbestos. 

Weight  of 

AgBr 

from 

filtrate. 

Corrected 
weight  of 
AgBr  in 
vacuum. 

Ratio 
2  AgBr: 
AgjCrO*. 

gm. 

gm. 

gm. 

gm. 

gm. 

gm. 

4 

I 

2.63788 

2.98579 

0.00028 

0.00056 

0.00014 

2.98621 

1. 13  205 

5 

II 

2.82753 

3.20018 

0.00008 

0.00060 

0.00014 

3.20084 

1. 13203 

6 

III 

2-33454 

2.64054 

0.00032 

0.00220 

0.00026 

2.64268 

I.13199 

7 

I 

1. 77910 

2.01304 

0.00050 

0.00144 

0.00004 

2.01402 

1. 13204 

8 

I 

2.33198 

2.63988 

0.00030 

0.00034 

0.00002 

2.63994 

1. 13206 

9 

II 

3.10402 

3-51311 

0.00033 

0.00094 

0.00018 

3-51390 

1. 13205 

10 

III 

2.92751 

3-31411 

0.00027 

0.00033 

0.000 10 

3-31427 

I.13211 

II 

III 

4.21999 

4-77677 

0.00055 

0.00126 

0.00014 

4.77762 

I.13214 

12 

II 

5-24815 

5-93939 

0.00025 

0.00170 

0.00020 

5.94104 

I. 13 203 

13 

IV 

6.24014 

7.06401 

0.00039 

0.00104 

0.00018 

7.06484 

I.13216 

14 

IV 

7-92313 

8.96913 

0.00083 

0.00129 

0.00022 

8.96982 

I.13211 

Ai 

/erage 

.  1.1^207 

Per  cent 

of  Ag  in  A 

gjCrO^    . 

.    .    .    65.0321 

Average 

per  cent  oi 

Ag  in  Ag2 

CrO*     .   . 

•    .    •    65.0333 

Richards  and  Wells:  loccit. 
Baxter:  loc  cit. 


THE  ANALYSIS  OF  SILVER  CHROMATE.  IJI 

DISCUSSION    OF    RESULTS. 

In  comparing  the  analytical  results,  it  is  to  be  noted  first  that  the  compositions 
of  the  different  samples  agree  within  less  than  o.oi  per  cent,  as  the  following 
averages  show. 

2  AgBr :  Ag2Cr04  2  AgCl :  Ag2CrO« 

Sample  I  1.13205  Sample  11        0.86409 

Sample  II  1. 13204  Sample  IV       0.86411 

Sample  III         1. 13 208 
Sample  IV         1.13214 

If  anything,  Samples  I  and  II  show  a  somewhat  lower  percentage  of  silver 
than  Samples  III  and  IV.  These  samples  were  made  from  ammonium  chromate 
which  contained  a  slight  excess  of  chromic  acid.  This  excess  of  acid  accumulated 
in  the  solution  during  the  precipitation  of  the  silver  chromate,  so  that  the  pre- 
cipitate formed  under  distinctly  acid  conditions,  although  the  acidity  was  not 
sufficient  to  present  any  danger  of  the  formation  of  dichromate.  Samples  III 
and  IV,  on  the  other  hand,  since  they  were  made  from  potassium  chromate, 
which  is  markedly  hydrolyzed,  were  formed  under  distinctly  basic  conditions, 
and  the  precipitation  or  occlusion  of  basic  salts  is  to  be  feared.  Such  occluded 
basic  salts  would  tend  to  raise  the  percentage  of  silver  in  the  chromate.  How- 
ever, Sample  IV  yielded  slightly  higher  results  than  Sample  III,  while  on  account 
of  the  method  of  precipitation  the  reverse  is  to  be  expected;  for  Sample  III  was 
precipitated  by  adding  the  silver  nitrate  to  the  chromate,  while  Sample  IV  was 
precipitated  by  adding  the  chromate  to  the  silver  solution,  the  mother-liquor 
remaining  neutral  in  both  cases.  Too  much  emphasis  should  not  be  laid  upon 
the  slight  apparent  difference  in  the  composition  of  the  different  samples  of 
salt,  since  the  variations  in  the  experiments  with  the  same  sample  are  as  large 
as  the  differences  between  the  samples.  Hence  the  average  result  from  the  dif- 
ferent samples  is  employed  in  the  final  calculations,  all  the  analyses  being 
given  equal  weight  in  each  series. 

In  addition  to  the  specimens  of  silver  chromate,  the  preparation  and  analysis 
of  which  have  been  described,  two  other  interesting  samples  were  prepared. 
One  was  formed  by  adding  a  0.04  normal  silver  nitrate  solution  to  a  solution 
of  chromic  acid  of  similar  concentration.  On  account  of  the  solubility  of  silver 
chromate  in  nitric  acid  solutions,  precipitation  was  only  partial.  The  precipi- 
tate was  washed  and  dried,  and  upon  analysis  was  f oimd  to  contain  so  little  silver 
that  the  presence  of  a  small  proportion  of  dichromate  was  certain,  a  result  which 
is  hardly  to  be  expected  in  the  Hght  of  Sherrill's  experiments. 

The  second  sample  was  prepared  by  heating  ammoniacal  solutions  of  silver 
chromate  in  platinum  vessels,  the  chromate  being  gradually  precipitated  as  the 
ammonia  was  expelled.  This  material  yielded  somewhat  irregular  results, 
which  on  the  whole  indicated  too  high  percentages  of  silver,  and  hence  the 
presence  of  basic  salts,  a  result  which  could  have  been  predicted  from  a  consid- 
eration of  the  conditions  of  preparation. 


152  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

It  is  to  be  noted  that  Series  I  and  Series  II  yield  percentages  of  silver 
differing  by  less  than  0.004  per  cent,  a  highly  satisfactory  agreement,  which 
indicates  purity  of  the  halogen  acids  employed  as  well  as  experimental 
accuracy. 

If  the  percentage  of  silver  in  silver  chromate  is  65.0333,  the  molecular  weight 
of  silver  chromate  may  be  calculated  from  the  atomic  weight  of  silver,  and  from 
the  latter  value  the  atomic  weight  of  chromium  by  difference.  These  calcula- 
tions are  carried  out  with  two  possible  assumed  values  for  the  atomic  weight  of 
silver,  oxygen  being  assumed  to  have  the  value  16.000.  It  is  to  be  noted  that 
the  percentage  error  in  the  determination  of  the  molecular  weight  of  silver 
chromate  is  multipHed  six  times  in  the  atomic  weight  of  chromium. 

If  Ag  =  107.880      Ag2Cr04  =  331.768    and    Cr  =  52.008 
If  Ag  =  107.870      Ag2Cr04  =  331-737    and    Cr  =  51.997 

Although  slightly  lower  than  the  previous  investigations,  these  results  agree 
with  them  as  closely  as  is  to  be  expected,  most  of  the  probable  errors  in  earher 
work  tending  to  make  the  results  too  high. 

The  more  important  results  of  this  research  may  be  briefly  summed  up  as 
follows: 

1.  Pure  silver  chromate  was  prepared. 

2.  It  is  shown  that  silver  chromate  can  not  be  completely  dried  without  de- 
composition. 

3.  The  proportion  of  residual  water  was  determined  in  salt  dried  at  definite 
temperatures. 

4.  The  specific  gravity  of  unfused  silver  chromate  is  found  to  be  5.625  at 
25°  C.  referred  to  water  at  4°  C. 

5.  The  per  cent  of  silver  in  silver  chromate  is  found  to  be  65.0333  by  two 
closely  agreeing  methods. 

6.  With  two  assumed  values  for  the  atomic  weight  of  silver  referred  to 
oxygen,  the  atomic  weight  of  chromium  is  found  to  have  the  following 
values: 

If  Ag  =  107.88  Cr  =  52.01 

If  Ag  =  107.87  Cr  =  52.00 

In  the  following  paper  the  analysis  of  silver  dichromate  is  described. 


X. 

A    REVISION    OF   THE  ATOMIC   WEIGHT    OF 

CHROMIUM. 

THE  ANALYSIS  OF  SILVER   BICHROMATE. 


By  Gregory  Paul  Baxter  and  Richard  Henry  Jesse,  Jr. 


Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  44,  421  (1909). 

Journal  of  tlie  American  Chemical  Society,  31,  541  (1909). 

Zeitschrift  fiir  anorganische  Chemie,  62,  331  (1909). 

Chemical  News,  100,  213,  228  (1909). 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF 
CHROMIUM. 

THE  ANALYSIS   OF  SILVER  DICHROMATE. 


INTRODUCTION. 


In  the  preceding  paper  is  described  a  successful  attempt  to  prepare  pure 
silver  chromate  and  to  determine  its  silver  content,  with  the  object  of  throwing 
light  upon  the  atomic  weight  of  chromium,  the  value  found  in  this  way,  52.01 
(Ag  =  107.88),  being  about  o.i  unit  lower  than  the  one  in  common  use.  The 
preparation  and  analysis  of  silver  dichromate  were  next  investigated.  Since  the 
proportion  of  chromium  in  the  dichromate  is  50  per  cent  larger  than  in  the 
chromate,  the  effect  of  experimental  uncertainty  upon  the  final  result  is  cor- 
respondingly reduced. 

Silver  dichromate  possesses  another  great  advantage  over  silver  chromate  for 
exact  work  in  that  it  may  be  readily  crystallized  from  nitric  acid  solutions,  and 
thus  may  be  freed  from  impurities  included  or  occluded  during  precipitation, 
with  the  exception  of  nitric  acid  and  moisture.  For,  the  silver  and  chromium 
being  present  in  equivalent  proportions  during  the  crystallization,  the  inclusion 
of  mother-Hquor  could  do  no  harm.  If  the  concentration  of  the  nitric  acid  is  suffi- 
ciently high,  there  is  no  possibility  of  the  separation  of  silver  chromate  as  such 
during  this  crystallization,  since  Sherrill^  has  shown  that  silver  chromate  changes 
rapidly  into  silver  dichromate  under  nitric  acid  solutions  more  concentrated 
than  0.075  normal.  This  is  primarily  due  to  the  low  value  of  the  dissociation 
constant  of  the  second  hydrogen  of  chromic  acid,  which  has  been  found  by  Sher- 
rill  to  be  6  X  lo"^,  the  solubility  product  of  silver  chromate  being  9  X  lo"^, 
and  that  of  silver  dichromate  being  2  X  lo"^.  Sherrill  has  also  investigated  the 
part  which  the  hydrochromate  ion  plays  in  the  equilibrium  relations  of  chro- 
mates  and  dichromates  in  solution  and  has  found  the  following  equation  to 
hold: 

(Cr^Or)  _ 
(HCrOJ2  -  75 

Although  obviously  the  concentration  of  the  hydrochromate  ion  in  dichro- 
mate solutions  (in  a  0.1  molal  solution  of  potassic  dichromate  fifteen  per  cent 
of  the  salt  existing  as  hydrochromate)  is  always  considerable,  the  precipitation 
of  the  solid  phase  AgHCr04  seems  not  to  be  possible.  Sherrill  was  not  able  to 
find  any  indication  of  the  presence  of  this  salt  in  the  precipitate  formed  by 
adding  silver  nitrate  to  chromic  acid  in  nitric  acid  solution.    Furthermore, 

*  Jour.  Amer.  Chem.  Soc,  29,  1641  (1907). 

15s 


156  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

since  the  water  content  of  our  material  was  carefully  investigated,  the  presence 
of  hydrochromate  in  traces  could  do  no  harm;  for  the  latter  substance  upon 
sufficient  heating  would  yield  dichromate  and  water  according  to  the  following 
equation; 

2AgHCr04  =  AgaCraOr  +  HsO. 

Although  the  presence  of  polychromates  other  than  the  dichromate  seemed 
improbable,  their  absence  from  our  material  was  shown  by  crystallizing  silver 
dichromate  from  nitric  acid  of  different  concentrations.  Since  this  variation 
was  without  effect,  it  may  be  reasonably  supposed  that  more  highly  acid  salts 
than  the  dichromates  were  neither  precipitated  as  solid  phases  nor  occluded. 

PURIFICATION    OF   MATERIALS. 

Only  slight  changes  were  made  in  the  methods  of  purifying  the  materials  used 
in  the  various  preparations  of  silver  dichromate  and  in  the  analyses  from 
those  described  in  the  preceding  paper. 

Nitric  acid  was  freed  from  chlorine  by  several  distillations  through  a  platinum 
condenser. 

Hydrochloric  acid  also,  after  dilution,  was  purified  by  distillation  with  a 
quartz  condenser. 

Hydrobromic  acid  was  prepared  from  bromine  which  had  been  twice  distilled 
from  solution  in  potassium  bromide,  the  bromide  in  the  second  distillation  being 
essentially  free  from  chlorine.  The  hydrobromic  acid  was  sjoithesized  by  passing 
carefully  cleansed  hydrogen  (made  from  the  lead-sodium  alloy  "hy drone"  and 
water)  through  the  bromine  at  about  40°  and  then  over  hot  platinized  asbestos, 
the  acid  being  collected  in  pure  water.  Iodine  was  eliminated  from  the  acid  by 
boiling  with  free  bromine  several  times.  Finally  the  acid  was  redistilled 
through  a  quartz  condenser  three  times  with  rejection  of  the  extreme  frac- 
tions. The  acid,  diluted  to  normal  concentration,  was  kept  in  a  well  protected 
glass  bottle. 

Silver  nitrate  was  prepared  from  silver  which  had  been  precipitated  once  as 
chloride,  and  then  reduced  with  invert  sugar.  The  nitric  acid  solution  of  the 
fused  product  was  evaporated  to  crystallization,  and  the  salt  was  then  three 
times  more  crystallized  from  nitric  acid  solutions,  the  crystals  being  drained 
centrifugally  in  a  centrifugal  machine  employing  platinum  Gooch  crucibles  as 
baskets.^  Heating  was  carried  out  over  electric  stoves  in  order  to  avoid  con- 
tamination by  the  combustion  products  of  illuminating  gas,  both  in  this  and 
in  all  other  preparations  in  this  research. 

The  best  commercial  potassium  dichromate  was  crystallized  four  times, 
once  from  aqueous  solution  in  Jena  glass,  and  three  times  in  platiniun  vessels. 

Chromic  acid  was  three  times  recrystallized  in  platinum  vessels  as  described 
in  the  preceding  paper. 

^  B&xtti:  Jour.  Amer.  Chetn.  Soc,  30,  286  (1908). 


THE  ANALYSIS   OF   SILVER  BICHROMATE.  1 57 

PREPARATION    OF    SILVER    BICHROMATE. 

Silver  dichromate  was  prepared  by  combining  either  potassium  dichromate  or 
chromic  acid  with  silver  nitrate  in  nitric  acid  solution  in  platinimi  vessels.  Pre- 
cipitation was  carried  out  in  fairly  concentrated  solution,  since  in  the  subsequent 
crystallization  of  the  silver  salt  from  nitric  acid  solution  any  included  substance 
was  sure  to  be  eliminated.  Although  the  inclusion  of  nitric  acid  during  the 
crystallization  was  to  be  feared,  and  was  actually  found  to  have  taken  place,  a 
method  was  devised  for  the  determination  of  this  nitric  acid,  together  with  the 
moisture  retained  by  the  solid. 

Sample  I.  —  Silver  nitrate  and  potassium  dichromate  were  dissolved  in  equiv- 
alent proportions  in  3  normal  nitric  acid,  the  concentration  of  each  salt  being 
about  0.7  normal.  The  cold  silver  nitrate  solution  was  added  very  slowly,  with 
constant  vigorous  stirring,  to  the  dichromate  solution.  After  the  precipitate 
had  been  allowed  to  settle,  the  mother-liquor  was  decanted,  and  the  precipitate 
was  centrif ugally  drained  and  rinsed  with  3  normal  nitric  acid  in  the  centrifugal 
machine. 

The  salt  was  then  five  times  recrystallized  from  solution  in  3  normal  nitric 
acid  with  centrifugal  drainage  after  each  crystallization.  Owing  to  the  slight 
solubility  of  silver  dichromate  in  nitric-acid  solutions  the  following  scheme  of 
crystalhzation  was  adopted.  The  dichromate  was  heated  with  the  nitric  acid 
solution  upon  the  electric  stove  until  the  acid  was  saturated  with  silver  di- 
chromate. Then  the  hot  solution  was  decanted  into  a  dish  through  a  platinum 
Gooch  crucible  without  a  mat  of  any  sort  but  with  small  holes,  in  order  to  re- 
move particles  of  silver  dichromate  either  suspended  in  the  solution  or  floating 
on  the  surface.  These  particles  were  always  of  considerable  size,  so  that  the 
resulting  solution  was  clear.  After  the  saturated  solution  had  cooled  and  had 
deposited  the  greater  part  of  its  charge  of  salt,  the  mother-liquor  was  contin- 
uously used  to  dissolve  fresh  portions  of  salt.  About  i  Hter  of  acid  was  used 
for  the  crystalhzation  of  about  50  gm.  of  dichromate.  Although  by  this  method 
the  impurities  in  the  original  salt  accumidate  in  the  mother-liquor,  on  account 
of  the  relatively  large  volume  of  the  mother-liquor,  there  was  little  danger  of 
these  impurities  being  carried  into  the  second  crop  of  crystals.  It  was  shown, 
for  instance,  that  the  mother-liquor  from  the  third  crystallization  was  free 
from  potassiiun.  This  mother-liquor  was  evaporated  to  small  bulk,  neutralized 
with  ammonia,  and  reduced  and  precipitated  with  hydrogen  sulphide.  The 
filtrate  after  evaporation  and  expulsion  of  the  ammonium  salts  gave  no  spec- 
troscopic flame  test  for  potassium. 

The  silver  dichromate  was  not  allowed  to  come  in  contact  with  water  or  any 
solution  except  the  3  normal  nitric  acid  solution. 

All  of  the  above  operations  were  carried  out  in  platinum  vessels. 

Sample  II.  —  This  sample  was  made  exactly  as  in  the  case  of  Sample  I,  ex- 
cept that  chromic  acid  was  employed  instead  of  potassium  dichromate,  and 
that  both  precipitation  and  crystallization  took  place  from  0.8  normal  nitric 
acid.    The  silver  dichromate  was  crystallized  five  times. 


158  RESEARCHES   UPON  ATOMIC  WEIGHTS. 

Sample  III.  —  The  most  dilute  nitric  acid  which  was  used  in  the  preparation 
of  the  silver  dichromate  was  about  0.16  normal,  solutions  of  this  concentration 
being  employed  in  the  precipitation  and  crystallization  of  Sample  III.  This 
sample  was  made  from  chromic  acid  and  silver  nitrate,  and  was  six  times  crys- 
taUized  from  0.16  normal  nitric  acid. 

The  chief  difference  in  the  purification  of  the  three  specimens,  aside  from  the 
concentration  of  acid  used  in  their  preparation,  lies  in  the  fact  that  Sample  I  was 
prepared  from  recrystallized  potassium  dichromate  and  II  and  III  from  chromic 
acid.    All  three  samples  were  crystallized  many  times  as  silver  dichromate. 

After  the  final  drainage  in  the  centrifugal  apparatus,  the  crystals  were  dried 
in  an  electric  oven  at  150°  for  several  hours.  Then  they  were  powdered  gently 
in  an  agate  mortar  and  kept  in  platinum  vessels. 

DRYING  OF   SILVER   DICHROMATE. 

In  preparing  the  silver  dichromate  for  analysis,  the  complete  elimination  of 
moisture  by  fusion  of  the  salt  was  impossible,  owing  to  the  ease  with  which  silver 
dichromate  decomposes.  Even  at  the  comparatively  low  temperature  of  the 
melting  point  of  the  dichromate,  about  400°,  oxygen  is  given  off  rapidly,  while 
at  temperatures  considerably  below  this  point,  300°,  and  to  a  very  slight  extent 
at  250°,  there  seemed  to  be  evidence  of  decomposition,  since  salt  heated  to  these 
temperatures  did  not  give  an  absolutely  clear  solution  in  dilute  nitric  acid.  In 
order  to  be  on  the  safe  side,  the  drying  of  the  salt  took  place  at  200°  C. 

The  heating  of  the  dichromate  was  effected  much  as  described  in  the  preced- 
ing paper  in  the  case  of  silver  chromate.  The  salt,  contained  in  a  weighed 
platinum  boat,  was  heated  in  a  current  of  pm-e  dry  air  in  a  hard-glass  tube  for 
4  hours  at  200°  C,  the  air  being  purified  and  dried  by  passing  over  hot  copper 
oxide,  soUd  potassic  hydroxide,  concentrated  sulphuric  acid  containing  dichro- 
mate, and  resublimed  phosphorus  pentoxide  successively.  The  oven  composed 
of  solid  aluminum  blocks  (page  78)  was  used,  by  means  of  which  the  tempera- 
ture could  be  maintained  constant  within  a  very  few  degrees. 

DETERMINATION    OF   SILVER   IN    SILVER    DICHROMATE. 

After  the  boat  had  been  allowed  to  cool  in  the  tube,  it  was  transferred  to  the 
weighing-bottle  by  means  of  the  bottling  apparatus  (page  8),  and  was  re- 
weighed.  Then  the  dichromate  was  transferred  to  a  flask  and  was  dissolved  in 
hot  0.8  normal  nitric  acid,  the  boat  and  the  weighing-bottle  being  carefully 
cleansed  with  nitric  acid  and  the  rinsings  being  added  to  the  main  solution.  The 
solution,  which  was  always  perfectly  clear,  was  quantitatively  transferred  to  the 
3-liter  glass  stoppered  precipitating  flask,  and  at  a  dilution  of  about  i  liter  was 
reduced  by  the  addition  of  a  very  slight  excess  of  sulphur  dioxide.  When  the 
solution  was  cold,  a  sHght  excess  of  hydrobromic  acid  was  diluted  to  about 
800  c.c.  and  then  was  slowly  added  to  the  silver  solution  with  continual  agitation. 
The  flask  was  stoppered  and  vigorously  sihaken.  After  24  hours'  standing  the 
flask  was  again  shaken,  and  then  was  allowed  to  stand  2  days  or  more,  until  the 
supernatant  solution  was  clear. 


THE  ANALYSIS   OF   SILVER  DICHROMATE.  1 59 

Next  the  silver  bromide  was  washed  at  least  eight  times  by  decantation  with 
pure  water  and  collected  upon  a  weighed  Gooch  crucible.  Then  it  was  dried 
in  an  electric  oven,  first  at  ioo°  for  2  hours,  then  at  175°  for  about  18  hours. 
After  cooHng  in  a  desiccator  near  the  balance  for  several  hours,  the  weight  of 
the  silver  bromide  was  determined. 

The  use  of  an  asbestos  mat  in  the  Gooch  crucible  made  it  necessary  to  collect 
and  determine  the  fibers  detached  during  the  filtration.  This  was  done  by 
passing  the  entire  filtrate  and  wash-waters  through  a  small  filter  paper.  The 
paper  was  ignited  in  a  weighed  porcelain  crucible,  and  the  ash  was  treated  with 
nitric  acid  and  then  hydrobromic  acid  to  convert  a  trace  of  reduced  silver  to  the 
state  of  bromide.  In  order  to  avoid  any  danger  from  adsorption  of  chromic 
salts  by  the  filter  paper,  at  the  end  of  the  filtration  the  paper  was  rinsed  with 
hot  dilute  hydrobromic  acid.  The  correction  for  asbestos  could  have  been 
avoided  if  it  had  been  possible  to  employ  a  Gooch-Munroe-Neubauer  crucible 
with  a  mat  of  platinum  sponge.  It  has  already  been  shown,  however,  in  the 
preceding  paper  (page  146),  that  such  crucibles  lose  markedly  in  weight  when 
exposed  to  the  action  even  of  the  dilute  aqua  regia  of  the  mother-liquors  of 
these  analyses. 

The  moisture  retained  by  the  silver  bromide  was  foimd  by  fusing  the  dried 
salt  in  a  porcelain  crucible,  the  loss  in  weight  on  fusion  being  determined.  The 
fused  silver  bromide  was  always  light  yellow  and  gave  every  indication  of  purity. 

As  in  the  preceding  research  a  small  quantity  of  silver  bromide  dissolved  in 
the  filtrate  and  wash-waters  was  found  by  evaporating  the  combined  filtrate 
and  wash-waters  until  nearly  all  the  excess  of  acid  had  been  expelled,  and  then, 
after  slight  dilution,  precipitating  the  silver  as  sulphide.  The  sulphide  was  col- 
lected on  a  small  paper,  the  ash  of  which,  after  ignition,  was  treated  with  nitric 
acid.  The  amount  of  silver  thus  obtained  was  found  by  comparing  in  a  nephel- 
ometer  precipitates  of  silver  bromide  produced  in  this  solution  and  in  very 
dilute  standard  solutions  of  silver. 

In  analysis  9  the  silver  was  precipitated  as  silver  chloride,  the  only  other  dif- 
ference in  the  procedure  being  that  the  precipitate  was  washed  with  dilute 
hydrochloric  acid  instead  of  pure  water. 

DETERMINATION    OF   MOISTURE   AND   NITRIC   ACID   IN 
SILVER   DICHROMATE. 

Silver  dichromate  which  has  been  crystallized  from  nitric  acid,  after  being 
dried  at  200°,  contains  traces  of  both  nitric  acid  and  water.  Both  of  these  sub- 
stances can  be  expelled  from  the  salt  by  fusion,  although  slight  decomposition 
of  the  salt  takes  place  simultaneously.  Since  the  only  readily  volatile  sub- 
stance which  can  be  formed  by  the  decomposition  of  the  salt  is  oxygen  gas,  the 
problem  of  the  determination  of  the  moisture  and  nitric  acid  consisted  in  that 
of  absorbing  in  a  quantitative  fashion  the  water,  nitric  acid,  and  nitric  peroxide 
formed  by  decomposition  of  the  nitric  acid.  This  was  effected  by  passing  the 
current  of  air  containing  the  moisture  and  nitrogen  compounds  through  two 


l6o  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

weighed  U-tubes,  one  containing  a  concentrated  solution  of  potassium  hy- 
droxide and  solid  potassium  hydroxide  and  the  other  resublimed  phosphorus 
pentoxide.  The  air  current  passed  first  through  the  potassium  hydroxide  tube 
in  order  that  moisture  vaporized  from  the  hydroxide  might  be  retained  by  the 
pentoxide  tube.  That  the  absorption  of  oxides  of  nitrogen  was  complete  was 
shown  by  the  fact  that  no  test  for  nitric  acid  could  be  obtained  beyond  the  phos- 
phorus pentoxide  tube  either  with  moist  litmus  paper  or  with  diphenylamine. 
Since  the  three  samples  of  silver  dichromate  were  crystallized  from  nitric  acid 
of  different  concentrations,  it  was  necessary  to  make  separate  determinations 
of  the  moisture  and  nitric  acid  content  with  each  sample.  Extreme  purity  of 
material  was  unnecessary,  and,  as  rather  large  quantities  of  salt  were  desired, 
three  samples  were  prepared  from  ordinary  silver  nitrate  and  potassium  di- 
chromate and  then  were  crystallized  from  nitric  acid  of  the  concentrations  3 
normal,  0.8  normal,  and  0.16  normal,  respectively,  glass  vessels  being  em- 
ployed throughout. 

Weighed  portions  of  the  silver  dichromate  were  heated  for  4  hours  at  200° 
in  a  current  of  pure  dry  air  exactly  as  in  preparing  the  salt  for  the  silver  analy- 
ses. Then  the  weighed  potassium  hydroxide  and  phosphorus  pentoxide  tubes 
were  attached  to  the  hard-glass  tube,  with  a  protection  tube  containing  phos- 
phorus pentoxide  at  the  end.  The  silver  dichromate  was  gradually  heated 
to  complete  fusion,  and  the  air  current  was  allowed  to  pass  through  the  sys- 
tem for  30  minutes  in  order  to  make  certain  that  all  the  vapors  expelled  from  the 
dichromate  were  carried  into  the  absorbing  tubes.  The  absorption  tubes  were 
then  reweighed. 

Before  the  tubes  were  weighed,  they  were  carefully  wiped  with  a  clean 
damp  cloth  and  were  allowed  to  stand  near  the  balance  case  for  one  hour.  The 
tubes  were  provided  with  ground  glass  stopcocks  lubricated  with  Ramsay 
desiccator  grease.  During  the  weighing  one  stopcock  in  each  tube  was  open  to 
equalize  the  air  pressure  within  and  without  the  tubes.  In  order  to  lessen  the 
error  in  weighing,  as  well  as  to  save  time  and  labor,  the  tubes  were  not  weighed 
separately,  but  together  as  one  system.  Counterpoise  tubes  of  the  same  shape 
and  size  were  always  employed.  Blank  determinations  showed  that  the  air 
current  and  manipulation  of  the  tubes  caused  an  increase  in  weight  of  o.oooio 
gm.  in  30  minutes.    This  quantity  is  applied  as  a  correction  in  every  case. 

In  place  of  a  platinum  boat  a  superficially  oxidized  copper  boat  was  used  in 
these  experiments.  At  the  low  temperature  of  fusion  of  silver  dichromate  there 
is  little  danger  of  decomposition  of  nitric  acid  or  oxides  of  nitrogen  by  the  oxi- 
dized copper.  It  is  to  be  noted  that  if  the  nitric  acid  is  decomposed  during  the 
experiment  according  to  the  following  equation: 

2HNO3  =  H2O  +  2NO2  +  O, 

and  is  absorbed  by  the  potassium  hydroxide  as  NO2,  there  is  a  slight  loss  of  oxy- 
gen. The  proportion  of  nitric  acid  present  being  very  small,  however,  this 
error  could  have  no  appreciable  effect  on  the  results. 


THE   ANALYSIS   OF   SILVER  BICHROMATE. 


i6i 


Sample. 

Weight  of 
AgjCrjOT 

Gain  in  weight 

of  absorption 

tubes. 

Gain 

Wt.  of  Ag^CraOi 

I 
I 

I 

22.52 
20.74 
12.25 

0.00448 
0.00378 
0.00235 

0.000194 
0.000177 
0.000184 

Avera^ 

;e 0.000186 

II 
II 
II 
II 

13-13 
15-91 
21-35 
19.60 

0.00309 
0.00317 
0.00391 
0.00373 

0.000235 
0.000193 
0.000178 
0.000185 

Average,  rejecting  the  first  determination,  0.000186 

III 
III 

20.89 
19.94 

0.00353 
0.00348 

0.000164 
0.000169 

Averag 

e     0.000167 

It  is  somewhat  surprising  that  Samples  I  and  II  contain  the  same  proportion 
of  volatile  matter.  This  agrees  with  the  result  of  the  sUver  determinations, 
however,  the  samples  proving  to  be  otherwise  very  similar.  As  is  to  be  ex- 
pected, Sample  III  contains  less  impurity  than  either  of  the  other  two. 

The  negative  corrections  as  found  above  are  applied  to  all  the  final  weights 
of  silver  dichromate  given  in  the  table  of  analyses. 

SPECIFIC    GRAVITY   OF   SILVER    DICHROMATE. 

The  specific  gravity  of  silver  dichromate  has  been  found  by  Schroder  ^  to  be 
4.669,  but  on  account  of  the  uncertainty  of  most  of  the  older  specific  gravity 
determinations  this  constant  was  very  kindly  redetermined  for  us  by  Mr. 
Victor  Cobb.  The  silver  dichromate  was  precipitated  from  dilute  nitric  acid 
solution  and  once  recrystaUized  from  normal  nitric  acid.  Then  it  was  dried  at 
200°  for  many  hours.  The  determination  was  effected  by  displacement  of  toluol 
of  specific  gravity  0.86218.  Care  was  taken  to  extract  entangled  air  from  the 
crystals  by  exhausting  the  air  from  the  pycnometer  in  a  vacuum  desiccator. 

The  Specific  Gravity  op  Silver  Dichromate. 


Weight  of  AgaCrjO? 
in  vacuum. 

Weight  of  toluol 
displaced  in  vacuum. 

Specific  gravity  of 
AgjCrjO, 

gm. 
29.308 
25-330 

gm. 

5-299 
4.578 

4.769 
4.770 

1  Liehig's  Jahresb.,  1879,31. 


l62  RESEARCHES   UPON  ATOMIC   WEIGHTS. 

The  following  vacuum  corrections  were  applied: 


Specific  gravity. 

Vacuum  correction. 

Weights     ....,- 

8.3 

0.862 

4.770 

6.473 

5.56 

+0.00126 
+0.000107 
+0.000041 
+0.000071 

Toluol 

Silver  dichromate 
Silver  bromide 
Silver  chloride 

A  No.  10  Troemner  balance  easily  sensitive  to  0.02  mg.  was  used  in  all  the 
weighings.  The  gold-plated  weights  were  carefully  standardized  to  hundredths 
of  a  milligram  by  the  method  described  by  Richards.^ 

Weighing  was  always  carried  out  by  substitution,  with  the  use  of  a  counter- 
poise as  nearly  as  possible  like  the  object  weighed,  both  in  material,  shape 
and  volume. 

Series  III.    2AgBr:  Ag2Cr207. 

Ag 


AgBr 


=  0.574453'' 


No.  of 
anal- 
ysis. 


Sample 
of 

AgzCrjO? 


II 

II 

II 

III 

III 

III 

I 

I 

III 


Corrected 
weight  of 
AgjCrzOT. 


gm. 

5-71554 
4.87301 
7-45476 
4.75269 
8.15615 
6.15412 
6.83662 

5-39883 
6.26657 


Total     .  .   55-60829 


Weight  of 
AgBr  in 
vacuum. 


gm. 
4.97107 
4.23870 
6.48380 
4.13409 
7.09477 
5-35306 
5-94656 
4.69610 
4.16034= 


Weight  of 
asbestos. 


gm. 

0.00024 
0.00019 
0.00034 
0.00020 
0.00022 
0.00007 
0.00030 
0.00027 
0.00018 


Dissolved 

AgBr  from 

filtrate. 


gm. 

0.00025 
0.00003 
0.00019 
0.00003 
0.00005 
0.00007 
0.00009 
0.00007 
0.00040 


Loss  on 
fusion. 


gm. 
0.00007 
0.00004 
0.00008 
0.00012 
0.00009 
o. 0001 1 
0.00017 
0.00013 
0.00016 


Corrected 
weight  of 
AgBr  in 
vacuum. 


gm. 

4.97149 
4.23888 
6.48425 
4.13420 
7-09495 
5-35309 
5.94678 
4.69631 
4.16076^ 


Ratio 
2.'VgBr: 


20 


0.869813 
0.869865 
0.869890 
0.869839 
0.869842 


0.8699034 


Average 0.869857 

48.37126     0.869854 


Average  from  Sample  I      0.869859 

Average  from  Sample  II 0.869834 

Average  from  Sample  III 0.869874 

Average 0.869856 

Per  cent  of  Ag  in  AgaCr207,  if  2 AgBr:  Ag2Cr207 

=  0.869857:  i.oooooo 49.9692 


1  Jour.  Amer.  Chem.  Soc,  22, 144  (1900). 
"Baxter,  Loc.  cit. 
•AgCl. 

*  Cakulated  from  the  ratio  AgBr:  AgCl  =  131.0171:  100.0000. 
gm.  AgCl  =0=5-45131  gna-  AgBr. 


Baxter,  Loc.  cit.  4.16076 


THE  ANALYSIS   OF  SILVER  DICHROMATE.  163 

The  preceding  table  gives  the  results  of  all  the  final  experiments  in  the  order 
in  which  they  were  carried  out.  The  preliminary  analyses,  which  were  de- 
fective in  various  ways,  are  not  recorded. 

DISCUSSION    OF   RESULTS. 

The  results  of  the  foregoing  experiments  are  as  concordant  as  one  can  rea- 
sonably expect,  since  the  insoluble  silver  salts  are  in  general  difficult  to  obtain 
definite  in  composition.^  The  extreme  values  differ  by  only  o.oi  per  cent, 
while  the  averages  of  the  different  samples  show  an  extreme  difference  of  less 
than  0.005  P^r  cent.  The  composition  of  the  dichromate  is  evidently  not  affected 
by  the  concentration  of  the  nitric  acid  from  which  it  is  crystalKzed,  since  the 
averages  from  the  different  samples  do  not  vary  regularly  with  the  concen- 
tration of  the  nitric  acid,  the  average  result  obtained  from  Sample  II  being 
lower  than  that  of  either  Sample  I  or  Sample  III. 

If  the  per  cent  of  silver  in  silver  dichromate  is  49.9692,  the  molecular  weight 
of  silver  chromate  may  be  calculated  from  the  atomic  weight  of  silver,  and 
from  the  molecular  weight  of  the  chromate  the  atomic  weight  of  chromiimi  by 
difference.  These  calculations  have  been  made  with  two  values  for  the  atomic 
weight  of  silver,  oxygen  being  assumed  to  have  the  value  16.000.  It  is  to  be 
noted  that  the  percentage  error  in  the  determination  of  the  molecular  weight 
of  silver  dichromate  is  multiplied  four  times  in  the  atomic  weight  of  chromium. 

If  Ag  =  107.880        Ag2Cr207  =  431.786        and  Cr  =  52.013 
If  Ag  =  107.870        Ag2Cr207  =  431-746        and  Cr  =  52.003 

In  the  following  table  are  given  the  results  of  the  preceding  research  upon 
silver  chromate  by  Baxter,  Mueller,  and  Hines,  together  with  the  average  of 
their  values  and  those  presented  in  this  paper: 

Baxter,  Mueller,  and  Hinea.  Average. 

If  Ag  =  107.880  Cr  =  52.008  52.011 

If  Ag  =  107.870  Cr  =  51-997  52.000 

The  agreement  of  the  two  independently  determined  values  is  highly  satis- 
factory, no  matter  which  value  for  the  atomic  weight  of  silver  is  assumed,  al- 
though the  higher  values  for  silver  give  slightly  better  agreement. 

The  atomic  weights  of  both  chromium  and  silver  may  be  calculated  indepen- 
dently of  any  assumption  except  the  atomic  weight  of  oxygen  from  the  fol- 
lowing equations: 

2Ag  +' Cr  +  64  =  °-*5°333  ,Ag  +  'cr  +  ii.  =  °-''99«9» 

to  be  52.074  and  107.941  respectively.  However  interesting  these  results  may 
be,  they  have  little  real  significance,  since  an  error  of  0.005  per  cent  in  either 
ratio  causes  an  error  of  over  o.i  unit  in  the  atomic  weights  of  both  silver  and 
chromium. 

*  Baxter  and  Coffin:  Proc.  Amer.  Acad.,  44,  179  (1909);  Jour.  Amer.  Chem.  Soc.,^l^  297; 
Zeit.  anorg.  Chem.,  62,  50.    See  also  preceding  paper. 


l64  RESEARCHES  UPON  ATOMIC  WEIGHTS. 

The  most  important  results  of  this  research  are  as  follows: 

1.  Pure  silver  dichromate  was  prepared. 

2.  It  is  shown  that  silver  dichromate  can  not  be  completely  dried  without 
decomposition. 

3.  It  is  shown  that  silver  dichromate  when  crystaUized  from  nitric  acid 
retains  traces  of  the  nitric  acid. 

4.  The  proportion  of  moisture  and  nitric  acid  in  silver  dichromate  treated  in 
definite  fashions  was  determined. 

5.  The  specific  gravity  of  silver  dichromate  is  found  to  be  4.770  at  25°  C. 
referred  to  water  at  4°  C. 

6.  The  per  cent  of  silver  in  silver  dichromate  is  found  to  be  49.9692. 

7.  With  two  assumed  values  for  the  atomic  weight  of  silver  referred  to  oxy- 
gen 16.000,  the  atomic  weight  of  chromium  is  found  to  have  the  following 
values: 

If  Ag  =  107.88  Cr  =  52.01 

If  Ag  =  107.87  Cr  =  52.00 

8.  If  these  results  are  averaged  with  those  previously  found  by  Baxter, 
Mueller,  and  Hines,  the  atomic  weight  of  chromium  is  found  to  be  as  follows: 

If  Ag  =  107.88  Cr  =  52.01 

If  Ag  =  107.87  Cr  =  52.00 


XI. 

A    REVISION   OF   THE   ATOMIC   WEIGHT   OF 
PHOSPHORUS.    . 

THE  ANALYSIS  OF  SILVER  PHOSPHATE, 


By  Gregory  Paul  Baxter  and  Grinnell  Jones. 


Proceedings  of  the  American  Academy  of  Arts  and  Sciences,  45,  137  (1910). 

Journal  of  the  American  Chemical  Society,  32,  298  (1910). 

Zeitschrift  fiir   anorganische   Chemie,   66,  97  (1910). 

Chemical  News,  loi,  150,  161,  176,  184  (1910). 


Contributions  from  the  Chemical  Laboratory  of  Harvard  College. 


A    REVISION    OF   THE    ATOMIC    WEIGHT    OF 
PHOSPHORUS. 

THE   ANALYSIS  OF  SILVER   PHOSPHATE. 


INTRODUCTION. 


Although  phosphorus  is  one  of  the  best  known  and  most  important  ele- 
ments, present  knowledge  concerning  its  atomic  weight  is  somewhat  inadequate. 
The  early  determinations  of  this  constant  by  Dulong,^  Pelouze,^  Berzelius,' 
and  Jacquelain*  are  widely  discrepant  and  have  no  particular  significance. 
Those  by  Schrotter,  Dumas,  van  der  Platts,  and  Berthelot,  on  the  other  hand, 
all  give  values  not  far  from  31.0,  and  this  value  has  been  selected  by  the  Inter- 
national Conmiittee  on  Atomic  Weights.  Although  these  investigations  have 
already  been  critically  discussed  by  Clarke,^  Brauner,^  and  others,  a  few  of  the 
more  important  sources  of  error  are  briefly  pointed  out  here. 

Schrotter,'  the  discoverer  of  red  phosphorus,  converted  weighed  quantities 
of  this  substance  into  phosphorus  pentoxide  by  combustion  in  a  stream  of 
oxygen.  As  the  mean  of  ten  determinations  which  varied  from  30.94  to  31.06, 
he  obtained  31.03  for  the  atomic  weight  of  phosphorus.  The  oxygen  used  was 
shghtly  moist,  as  Brauner  has  pointed  out,  since,  although  it  was  dried  by 
phosphorus  pentoxide,  it  was  finally  passed  through  a  tube  containing  calcium 
chloride!  The  phosphorus  pentoxide  formed  during  the  combustion  must  have 
retained  this  small  amount  of  water,  which  would  make  the  atomic  weight  of 
phosphorus  appear  too  low.  Schrotter  admits  that  the  combustion  was  incom- 
plete, and  since  this  error  would  tend  to  raise  the  atomic  weight  of  phosphorus, 
he  concludes  that  the  true  value  is  31.00. 

Dumas  ^  titrated  the  trichloride  of  phosphorus  against  silver  after  decompos- 
ing the  trichloride  with  water.  Since  the  sample  used  did  not  boil  at  constant 
temperature,  but  distilled  between  76°  and  78°,  it  must  have  been  impure.  If 
it  contained  oxychloride,  as  Clarke  has  suggested,  the  atomic  weight  of  phos- 
phorus would  be  found  too  high.  Dumas  overlooked  the  solubility  of  silver 
chloride  and  therefore  used  the  wrong  end-point  in  these  titrations.    Further- 

»  Ann.  Chim.  Phys.,  2,  149  (1816).  •  C.  R.,  20, 1053  (1845). 

»  Lehrbuch,  sth  Ed.,  3,  1188  (1845).  *  C.  R.,  33,  693  (1851). 

'  A  Recalculation  of  the  Atomic  Weights,  Smith.  Misc.  Coll.,  1910. 

'  Abegg,  Eandb.  der  anorg.  Chem.,  1907,  vol.  3,  part  3,  p.  366. 

'  Ann.  Chim.  Phys.  (3),  38,  131  (1853). 

*  Ann.  Chem.  Pharm.,  113,  28  (i860).  167 


1 68  RESEARCHES  UPON   ATOMIC  WEIGHTS. 

more  no  precautions  are  mentioned  either  for  preventing  access  of  water  to  the 
material  before  weighing  or  for  preventing  the  reduction  of  the  silver  salt  by 
the  phosphorous  acid  formed  in  the  decomposition  of  the  trichloride  with  water. 
Recalcvdated  on  the  basis  of  the  atomic  weight  of  silver  as  107.88,  his  five  analy- 
ses give  results  which  vary  between  30.99  and  31.08.    The  average  is  31.03. 

Van  der  Platts  ^  made  two  determinations  by  each  of  three  different  methods. 
He  obtained  the  values  30.90  and  30.97  by  the  precipitation  of  silver  from  silver 
sulphate  solution  with  phosphorus.  His  results  from  the  analysis  of  silver 
phosphate  were  31.08  and  30.95.  He  gives  no  details  of  the  method  of  prepar- 
ing and  analyzing  this  substance,  merely  making  the  statement,  "it  is  difi&cult 
to  be  sure  of  the  purity  of  this  salt."  Finally,  by  the  combustion  of  yellow 
phosphorus  in  oxygen  he  obtained  the  results  30.99  and  30.96.  The  very  meagre 
descriptions  of  these  experiments  preclude  criticism. 

Using  Leduc's  data  for  the  densities  and  compressibilities  of  phosphine  and 
oxygen,  Daniel  Berthelot  ^  has  calculated,  by  the  method  of  limiting  densities, 
the  molecular  weight  of  phosphine  to  be  34.00.  and  the  atomic  weight  of  phos- 
phorus to  be  30.98. 

Very  recently  Gazarian^  has  obtained  a  considerably  lower  value  for  the 
molecular  weight  of  phosphine,  33.93.  This  value  was  calculated  from  the  ex- 
perimentally determined  weight  of  the  standard  liter  by  the  four  methods  of 
molecular  volumes  (Leduc),  limiting  densities  (Berthelot),  critical  constants 
(Guye),  and  "indirect"  limiting  densities  (Berthelot).  The  different  methods 
give  essentially  identical  results,  except  in  the  case  of  the  direct  method  of  lim- 
iting densities.  By  the  latter  method  a  value  0.06  unit  higher  is  obtained,  but 
Gazarian  rejects  the  result  on  the  basis  of  inaccurate  knowledge  of  the  compres- 
sibiUty  of  phosphine.  It  is  highly  desirable  to  obtain  more  certain  knowledge 
of  the  compressibility  of  phosphine,  since  the  method  of  limiting  densities  is  the 
most  rehable  of  all  the  methods  for  applying  the  correction  to  the  densities  made 
necessary  by  deviations  from  the  laws  of  a  perfect  gas. 

The  other  methods  are  burdened  with  arbitrary  assumptions  aind  empirical 

constants,  and  moreover  Baume  ^  has  shown  that  both  the  method  of  molecular 

volumes  and  the  method  of  critical  constants  give  correct  results  only  with  gases 

Tc 
for  which  the  ratio  — ^  is  nearly  i,  whereas  for  phosphine  this  ratio  is  1.26. 

If  the  molecular  weight  of  phosphine  be  assumed  to  be  33.93,  the  atomic 
weight  of  phosphorus  is  30.91.  In  the  light  of  this  low  result  it  is  unfortunate 
that  Gazarian  prepared  phosphine  by  only  one  method,  and  that  he  did  not 
determine  the  purity  of  the  gas,  i.  e.,  by  absorption.  Gazarian  used  the  method 
of  Matignon  and  Trannoy,^  which  consists  in  heating  calcium  phosphate  and 
aluminum  together  imtil  they  react,  and  then  treating  the  product  of  this  re- 
action without  further  purification  with  water  in  a  gas  generator.    Matignon 

1  C.  R.,  100,  52  (1885).  *  C.  R.,  126,  141S  (1898). 

•  Jour,  de  Chim.  Phys.,  7,  337  (1909).  *  Jour,  de  Chim.  Phys.,  6, 76  and  86  (1908). 

^  C.  R.,  148, 167  (1909). 


A  REVISION   OF   THE   ATOMIC   WEIGHT   OF  PHOSPHORUS.  169 

and  Trannoy  show  that  the  gas  prepared  in  this  way  by  them  contained  about 
3  per  cent  of  hydrogen,  probably  derived  from  calcium  contained  by  the  phos- 
phide. In  this  case  some  calcium  nitride  would  be  formed,  since  the  phosphide 
was  made  in  air;  and  this  would  produce  ammonia  as  an  impurity  in  the  phos- 
phine.  Although  the  gas  was  purified  by  fractional  distillation,  according  to 
Gazarian's  statements  hydrogen  is  difficult  to  eliminate,  and  a  proportion  of 
only  0.4  per  cent  would  be  sufficient  to  lower  the  atomic  weight  of  phosphorus 
0.1  vmit.  Ammonia  would  be  even  more  difficult  to  remove,  since  its  boiling 
point  is  only  50°  higher  than  that  of  phosphine.  The  effect  of  a  given  percent- 
age of  impurity  is,  however,  much  less  with  ammonia  than  with  hydrogen, 
although  in  the  same  direction. 

From  the  preceding  brief  simimary  it  is  evident  that  the  imcertainty  in  the 
atomic  weight  of  phosphorus  is  as  great  as  0,1  unit,  and  that,  as  Brauner  re- 
marks at  the  conclusion  of  his  review  of  the  subject,  "a  revision  of  the  atomic 
weight  of  phosphorus  with  modem  means  is  urgently  necessary." 

The  analysis  of  silver  phosphate  was  selected  as  one  of  the  most  promising 
methods  of  attacking  the  problem,  since  the  per  cent  of  silver  can  be  determined 
exactly  by  a  method  which  has  been  carefully  studied,  especially  in  this  labora- 
tory. The  accuracy  of  the  result  will  therefore  depend  primarily  upon  the 
success  attained  in  preparing  silver  phosphate  in  a  perfectly  definite  and  pure 
state.  The  greater  part  of  the  following  research  was  devoted  to  the  solution  of 
this  problem,  which  van  der  Platts  found  so  difficult. 

The  analysis  of  the  halogen  compoimds  of  phosphorus  offers  certain  difficul- 
ties owing  to  the  ease  with  which  these  substances  are  decomposed  by  water, 
and  to  the  necessity  for  oxidizing  the  phosphorous  acid  resulting  from  the  de- 
composition of  the  halogen  compounds  with  water  before  the  addition  of  silver 
nitrate.  An  investigation  upon  the  tribromide  of  phosphorus  is  now  in  progress 
in  this  laboratory.  Phosphonium  compounds  were  found  utterly  unsuited  for 
exact  analysis  on  account  of  their  instability. 

PURIFICATION    OF   MATERIALS. 

Water.  —  All  the  water  used  in  this  research  was  made  from  the  laboratory 
supply  of  distilled  water  by  distillation,  first  from  an  alkaline  permanganate 
solution,  and  then,  after  the  addition  of  a  trace  of  sulphuric  acid,  through  a 
block  tin  condenser. 

Ammonia. — The  best  commercial  ammonia  was  distilled  into  the  purest 
water. 

Nitric  Acid.  — The  best  commercial  concentrated  acid  was  twice  fractionally 
distilled  through  a  platinum  condenser,  with  the  rejection  of  the  first  third  of 
the  distillate.  Every  sample  was  shown  to  be  free  from  chloride  by  careful 
nephelometric  tests. 

Hydrochloric  Acid.  —  The  best  commercial  C.  P.  acid,  diluted  with  an  equal 
volume  of  water,  was  distilled  through  a  platinum  condenser. 


lyo  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

Hydrohromic  Acid.  — This  substance  was  prepared  in  conjunction  with  Mr. 
F.  B.  Cofl&n,  who  was  engaged  in  a  parallel  research  upon  the  atomic  weight  of 
arsenic,  which  has  been  described  in  the  seventh  paper  of  this  collection.  Com- 
mercial bromine  was  converted  into-.potalssium  bromide  by  addition  to  recrys- 
tallized  potassium  oxalate.  In-  a-  concentrated  solution  of  this  bromide,  in  a 
distilling  flask  cooled  with  ice,  bromin'C  was  dissolved,  and  distilled  from  the 
solution  into  a  flask  cooled  with  ice.  A  portion  of  the  purified  bromine  was 
then  converted  into  potassium  ibfomide  with  pure  potassium  oxalate  as  before, 
and  the  remainder  of  the  bromine  was  distilled  from  solution  in  this  pure  potas- 
sium bromide.  The  product  obtained  was  thus  twice  distilled  from  a  bromide, 
the  bromide  in  the  second  distillation  being  essentially  free  from  chlorine.  This 
treatment  has  already  been  proved  sufficient  to  free  bromine  from  chlorine.^ 
Hydrobromic  acid  was  S5nithesized  from  the  pure  bromine  by  bubbling  hy- 
drogen gas  (made  by  the  action  of  water  on  "hy drone")  through  the  bromine 
warmed  to  4o°-44°  and  passing  the  mixed  gases  over  hot  platinized  asbestos 
in  a  glass  tube.  The  apparatus  was  constructed  wholly  of  glass.  The  hydrogen 
was  cleansed  by  being  passed  through  two  wash-bottles  containing  dilute  sul- 
phuric acid,  and  through  a  tower  filled  with  beads  also  moistened  with  dilute 
sulphuric  acid.  The  hydrobromic-acid  gas  Was  absorbed  in  pure  Water  contained 
in  a  cooled  flask.  In  order  to  remove  iodine  the  solution  of  hydrobromic  acid 
was  diluted  with  water  and  twice  boiled  with  a  small  quantity  of  free  bromines 
Then  a  small  quantity  of  recrystallized  potassium  permanganate  was  added 
to  the  hydrobromic  acid  solution,  and  the  bromine  set  free  was  expelled  by 
boiling.  Finally  the  acid  was  distilled  with  the  use  of  a  quartz  condenser,  the 
first  third  being  rejected.  It  was  preserved  in  a  bottle  of  Nonsol  glass  pro- 
vided with  a  ground-glass  stopper.  The  purity  of  the  hydrobromic  acid  was 
tested  by  a  quantitative  synthesis  of  silver  bromide.  The.  silver  used,  which 
was  kindly  furnished  by  Mr.  G.  S.  Tilley,  had  been  prepared  with  all  the  neces- 
sary precautions  for  work  on  the  atomic  weights  of  silver  and  iodine.^  The 
procedure  used  by  Baxter  ^  for  the  synthesis  of  silver  bromide  from  a  weighed 
amount  of  silver  was  followed  in  detail.  In  this  experiment  6.02386  gm.  of 
silver  yielded  10.48627  gm.  of  Silver  bromide;   hen.ce,  silver  rbr^mide  contains 

57.4452  per  cent  of  silver,  while  Baxter  found  as  thp,m,§an  oft jE§, determinations 

57.4453  per  cent.    The  hydrobromic  acid  was  evidently  pure., 

Silver  Nitrate.  —  Crude  silver  nitrate  was  reduced  with  ammonium  formate, 
made  by  passing  ammonia  gas  into  redistilled  formic  acid.  The  reduced  silver 
was  washed  with  the  purest  water,  until  the  wash- waters  no  longer  gave  a  test 
for  ammonia  with  Nessler's  reagent,  and  was  fused  ,on  sugar  charcoal.    The 

^  Baxter:  Proc.  Amer.  Acad.,  42,  201  (1906);  /o«r.  Amer.'Chem.  Soc,  28, 1322;  Zeit.  anorg. 
Chem.,  50,  389.     (See  page  60.) 

2  Baxter  and  T'lWey:  Jour.  Amer.  Chem.  Soc,  31,  201  (1909);  Zeit.  anorg.  Chem. ,61,  293. 
(See  page  120.) 

*  Baxter,  Loc.  cit. 


A  REVISION  OF  THE  ATOMIC  WEIGHT   OF  PHOSPHORUS.  171 

buttons  were  then  scrubbed  with  sea-sand  and  thoroughly  cleansed  with  am- 
monia and  nitric  acid.  They  were  then  dissolved  in  redistilled  nitric  acid,  in  a 
platinum  dish.  After  the  silver  nitrate  solution  had  been  evaporated  on  a  steam 
bath  until  satm-ated,  an  equal  volume  of  redistilled  nitric  acid  was  added  and  the 
solution  was  cooled.  The  precipitated  silver  nitrate  was  very  completely  drained 
in  a  centrifugal  machine,  provided  with  platinum  Gooch  crucibles  to  retain  the 
salt.^  A  similar  recrystaUization  followed.  The  final  product  was  preserved 
in  Jena  glass  vessels  under  a  bell- jar. 

Disodium  Phosphate. — One  kilogram  of  Merck's  best  disodium  phosphate  was 
dissolved  in  hot  water  in  a  porcelain  dish  and  hydrogen  sulphide  was  passed  into 
the  solution  for  several  hours.  After  standing  for  24  hours,  the  solution  was 
again  heated,  saturated  with  hydrogen  sulphide  and  filtered.  The  filtrate  was 
slightly  green,  owing  to  the  presence  of  iron.  The  solution  was  boiled  to  expel 
the  hydrogen  sulphide  and  a  small  amount  of  green  precipitate  filtered  out.  The 
filtrate  was  still  distinctly  green.  The  sodium  phosphate  was  then  crystallized 
fifteen  times,  five  times  in  porcelain  with  centrifugal  drainage  of  the  crystals 
in  a  large  porcelain  centrifugal  machine,  ten  times  in  platinum  vessels  with  cen- 
trifugal drainage  of  the  crystals  in  platinum  Gooch  crucibles.  The  green  color 
concentrated  in  the  first  mother-Hquor.  When  tested  by  means  of  the  Marsh 
test,  this  material  was  found  to  contain  only  a  mere  trace  of  arsenic,  which  was 
estimated  to  be  o.oi  mg.  in  lo  gm.  of  the  salt.  This  small  amount  could  have  no 
effect  on  the  analytical  results,  especially  since  the  percentage  of  silver  in  silver 
arsenate  is  nearly  the  same  as  in  silver  phosphate.  By  means  of  the  nephelo- 
meter  it  was  proved  that  this  material  contained  no  chloride  or  other  substances 
which  could  be  precipitated  by  silver  nitrate  in  the  presence  of  dilute  nitric 
acid. 

Sodium  Ammonium  Hydrogen  Phosphate.  —  The  best  commercial  micro- 
cosmic  salt  was  recrystallized  four  times  in  platiniun  vessels.  It  was  tested  for 
arsenic  by  Marsh's  method  with  negative  results  and  gave  no  opalescence  visi- 
ble in  the  nephelometer  when  tested  with  silver  nitrate  and  dilute  nitric  acid. 

PREPARATION    OF   TRISILVER    PHOSPHATE. 

Silver  phosphate  was  prepared  by  mixing  dilute  solutions  of  silver  nitrate 
with  solutions  of  sodium  and  ammonium  phosphates.  Since  it  is  not  feasible 
to  purify  silver  phosphate  by  recrystaUization,  the  conditions  of  precipitation 
must  be  so  chosen  that  a  pure  product  will  be  obtained  at  once. 

In  order  to  avoid  inclusion  and  occlusion  of  silver  nitrate,  sodium  nitrate, 
sodium  phosphate,  or  monosilver  or  disilver  phosphate,  all  of  the  solutions 
for  precipitation  were  made  about  0.03  N.  All  samples  after  precipitation  were 
thoroughly  washed  and  allowed  to  stand  in  water  for  at  least  24  hours,  in  order 
to  convert  occluded  acid  phosphates  into  trisilver  phosphate.    Qualitative  tests 

*  Baxter:  Jour.  Amer.  Chetn.  Soc,  30,  286  (1908). 


172 


RESEARCHES   UPON   ATOMIC   WEIGHTS. 


for  nitrate  with  diphenylamine  and  for  sodium  by  the  spectroscope  showed  that 
all  of  the  first  three  substances  named  could  be  completely  washed  out. 

Joly  ^  states  that  disilver  phosphate  is  stable  in  the  presence  of  phosphoric 
acid  containing  40  per  cent  (11.8  N)  of  phosphoric  anhydride,  but  is  trans- 
formed into  trisilver  phosphate  if  the  acid  contains  38  per  cent  (ii.o  N)  or 
less  of  phosphoric  anhydride.  Since  all  the  solutions  used  for  the  preparation 
of  silver  phosphate  Avere  nearly  neutral,  it  is  evident  that  the  precipitation  of 
disilver  phosphate  as  a  distinct  phase  in  equiUbrium  with  the  solution  is  not 
to  be  feared. 

It  is,  however,  not  such  a  simple  matter  to  prove  the  absence  of  occluded 
disilver  hydrogen  phosphate  or  monosilver  hydrogen  phosphate.  Much  Hght  is 
thrown  on  this  point  in  a  recent  paper  by  Abbott  and  Bray  ^  upon  the  dissocia- 
tion constants  of  the  three  hydrogens  of  phosphoric  acid,  which  were  found  to  be 
I.I  X  io'2,  1.95  X  10'''  and  3.6  X  10"^^  respectively.  Since  the  phosphate 
ion  (P04=)  is  almost  completely  hydrolyzed  to  the  monohydrophosphate  ion 
(HP04=),  even  in  slightly  alkaline  solutions,  and  since  in  slightly  acid  solutions 
the  dihydrophosphate  ion  (H2PO4 — )  acquires  an  appreciable  concentration,  the 
possibility  of  occlusion  must  be  examined  with  especial  care. 

The  concentrations  in  the  following  table  are  either  taken  directly  from  a 
table  given  by  Abbott  and  Bray  or  calculated  from  these  numbers  with  the 
help  of  the  values  of  the  dissociation  constants  of  phosphoric  acid.  The  values 
are  expressed  in  formular  weights  per  liter,  the  total  concentration  of  the  salt 
being  in  each  case  0.05. 


NaNH^HPOi 

NajNHjPOi 

H2P04- 

HP04= 

P04= 

OH- 
H+ 

0.001184^ 
0.032653 
0.0000016^ 
0.00000079  ^ 
0.0000000075  * 

0.000002  * 
0.03219 3 
0.001123^ 
0.000502  3 
0.000000000012* 

It  will  be  noted  that  the  replacement  of  the  remaining  hydrogen  in  sodium 
ammonium  hydrogen  phosphate  by  sodium  decreases  the  concentration  of  the 
hydrogen  ion  to  0.16  per  cent  of  its  value  in  the  microcosmic  salt  solution  and 
decreases  the  concentration  of  the  dihydrophosphate  ion  to  0.2  per  cent  of  its 
former  value.    The  concentration  of  the  monohydrophosphate  ion  remains 


1  C.  R.,  103,  1071  (1886).  2  Jour.  Amer.  Chem.  Soc,  31,  755  (1909). 

*  These  values  are  taken  directly  from  the  table  of  Abbott  and  Bray. 

*  These  values  are  calculated  from  the  others  in  the  above  table  by  the  aid  of  the  following 
equations: 

(H+)(PO,s)  ^  ,    ,  ^  ^„_i3  (H+)(HPO«=)  _ 

(HP04=)  3    ox  10  (HjPO^-) 


(H+)(OH-)  =  0.59  X  lo-W 


1.95  X  10  ~7 


A   REVISION   OF   THE   ATOMIC   WEIGHT   OF   PHOSPHORUS.  1 73 

essentially  unchanged,  while  the  concentration  of  the  phosphate  ion  is  increased 
seven  hundred  times.  Disodium  phosphate  doubtless  takes  a  position  inter- 
mediate between  the  other  two  solutions  in  this  regard,  since  it  is  more  alkaline 
than  microcosmic  salt  and  less  so  than  disodium  ammonium  phosphate.  The 
numbers  given  above  refer  to  solutions  which  are  five  times  as  strong  as  those 
used  in  this  research,  but  the  conditions  in  the  more  dilute  solutions  must  be 
very  similar.  Furthermore,  the  exact  values  have  no  great  importance,  as  the 
concentrations  of  the  various  ions  change  continuously  during  precipitation. 
It  is  evident  from  the  figures  given  above  and  from  the  value  of  the  dissociation 
constant  of  the  second  hydrogen  of  phosphoric  acid  that  if  the  concentration  of 
hydrogen  ion  increases  above  its  value  in  a  microcosmic  salt  solution,  the  con- 
centration of  the  dihydrophosphate  ion  must  increase  greatly  at  the  expense  of 
the  monohydrophosphate  ion.  If  there  is  any  tendency  for  the  occlusion  of 
disilver  hydrogen  phosphate  or  monosilver  hydrogen  phosphate,  the  amounts 
of  these  salts  occluded  would  be  expected  to  depend  on  the  concentration  of  the 
undissociated  molecules  of  these  salts  in  the  solution,  and  therefore  on  the 
concentration  of  the  silver  ion  and  on  the  concentration  of  the  monohydrophos- 
phate or  dihydrophosphate  ion  respectively. 

The  exact  concentrations  of  the  ions  during  the  precipitation  can  not  be 
calculated,  since  the  solubility  of  silver  phosphate  in  slightly  acid  solutions  and 
the  solubility-product  of  silver  phosphate  are  not  known.  It  is,  however,  easy 
to  understand  from  a  study  of  the  conditions  under  wliich  the  various  samples 
of  silver  phosphate  were  precipitated,  that  these  concentrations  must  have 
varied  greatly  in  the  preparation  of  the  different  samples  and  therefore  con- 
stancy of  composition  gives  a  strong  presumption  that  there  is  very  little  or  no 
tendency  for  the  occlusion  of  the  undesired  acid  salts. 

Samples  N  and  0.  —  A  0.03  normal  solution  of  silver  nitrate  was  slowly 
poured  into  a  0.03  normal  solution  of  disodium  hydrogen  phosphate  with  fre- 
quent shaking.  This  reaction  may  be  roughly  considered  to  take  place  in  two 
stages  represented  by  the  equations 

sAgNOg  +   2Na2HP04  =  Ag3P04  +  NaH2P04  +  3NaN03 
3AgN03  +   NaH2P04  =  AgsPOi  +  NaNOa  +  2HNO3 

At  the  beginning  of  the  precipitation  the  solution  is  very  sKghtly  alkaline  and 
remains  very  nearly  neutral  during  the  addition  of  the  first  half  of  the  silver 
nitrate.  The  concentration  of  the  silver  ion  is  kept  very  low  by  the  excess  of 
phosphate  and,  therefore,  little  occlusion  of  the  acid  salts  is  to  be  expected  in 
spite  of  the  fact  that  the  solution  contains  appreciable  concentrations  of  the 
monohydrophosphate  and  dihydrophosphate  ions.  The  precipitate  during  this 
stage  is  very  finely  divided  and  does  not  settle  well  and,  therefore,  no  attempt 
was  made  to  collect  it  separately. 

During  the  addition  of  the  second  half  of  the  silver  nitrate  the  solution  be- 
comes slightly  acid  and  the  solubility  of  the  silver  phosphate  increases  rapidly. 


174  RESEARCHES   UPON  ATOMIC  WEIGHTS. 

The  precipitate  settles  readily.  During  the  second  stage  the  conditions  are  more 
favorable  for  the  occlusion  of  the  acid  phosphate,  but  only  a  small  amount  of 
silver  phosphate  is  precipitated  during  this  stage. 

After  standing  a  short  time  the  mother-Hquor  was  decanted  from  the  pre- 
cipitate, and  exactly  the  calculated  amount  of  redistilled  ammonia,  diluted  to 
I  liter,  was  added  to  neutraHze  the  excess  of  acid  and  complete  the  precipitation. 
Since  this  sample  was  evidently  produced  from  a  solution  which  was  slightly 
acid  at  the  beginning  of  the  precipitation,  although  very  nearly  neutral  at  the 
end,  and  since  it  contained  a  considerable  amount  of  silver,  the  conditions  were 
favorable  for  the  formation  of  acid  salts. 

Both  precipitates  were  transferred  to  a  large  platinum  dish  and  washed  many 
times  by  decantation  with  the  purest  water.  This  washing  was  prolonged  over 
more  than  24  hours  in  order  to  give  time  for  all  soluble  matter  to  be  leached 
out.  When  the  precipitates  were  tested  for  nitrate  with  diphenylamine,  negative 
results  were  obtained.  Sodium  was  found  to  be  absent  by  spectroscopic  tests. 
The  precipitates  were  drained  as  far  as  possible  in  a  platinum  centrifugal 
machine,  and  the  drying  was  completed  by  heating  in  platinum  crucibles  in  an 
electric  air  bath  for  several  hours,  first  at  90°  and  finally  at  about  130°.  The 
dried  lumps  of  silver  phosphate  were  then  gently  ground  in  an  agate  mortar. 
The  samples  were  preserved  in  platinum  crucibles  over  sulphuric  acid  in  the 
dark.    All  of  the  operations  were  performed  in  a  dark  room. 

The  sample  prepared  by  pouring  silver  nitrate  into  disodium  phosphate  is 
designated  Sample  N,  and  the  sample  prepared  by  adding  ammonia  to  the 
mother-liquors  is  designated  Sample  0. 

Sample  P.  —  A  0.3  normal  solution  of  disodiima  ammonium  phosphate  was 
prepared  by  dissolving  a  weighed  amount  of  disodiimi  hydrogen  phosphate  and 
then  adding  the  calculated  amount  of  redistilled  ammonia.  The  solution  was 
then  slowly  poured  into  a  0.03  normal  solution  of  silver  nitrate.  By  this  method 
of  precipitation  the  solution  is  maintained  as  nearly  neutral  as  is  possible,  be- 
cause the  excess  of  silver  prevents  the  concentration  of  phosphate  in  solution 
from  exceeding  a  very  small  value,  so  that  neither  can  the  solution  become 
alkaline  by  hydrolysis  nor  can  the  concentration  of  hydrophosphate  attain  an 
appreciable  value.  The  absence  of  the  hydrophosphate  ions  would  be  expected 
to  prevent  the  formation  and  occlusion  of  acid  silver  phosphate  in  this  sample 
whereas  in  Sample  N  the  same  result  is  probably  brought  about  by  the  absence 
of  the  silver  ion.  Unfortunately  both  of  these  favorable  conditions  can  not  be 
combined  in  one  precipitation,  as  will  be  shown  later.  This  precipitate 
settled  readily.  The  washing,  testing,  and  drying  were  carried  out  as  already 
described  for  Samples  N  and  O.    This  sample  is  designated  Sample  P. 

Sample  R.  —  A  0.03  normal  solution  of  sodivma  ammonium  hydrogen  phos- 
phate was  slowly  poured  into  a  similar  solution  of  an  equivalent  amount  of 
silver  nitrate.  Under  these  conditions  the  solution  contains  an  excess  of  silver, 
which  tends  to  produce  occlusion  of  acid  phosphates,  since  the  solution  becomes 


A  REVISION   OF  THE  ATOMIC  WEIGHT   OF  PHOSPHORUS.  175 

more  and  more  acid  as  the  precipitation  proceeds,  and  as  the  precipitation  is 
therefore  far  from  complete,  the  concentrations  of  the  two  hydrophosphate 
ions  gradually  approach  a  very  considerable  value.  At  no  stage  could  the  solu- 
tion become  alkaline  by  hydrolysis.  It  should  be  noticed  that  the  procedure 
dififers  from  that  used  in  preparing  Sample  N  in  that  the  precipitate  is  formed 
in  the  presence  of  an  excess  of  silver  nitrate  instead  of  an  excess  of  phosphate, 
and  that  this  difference  in  the  method  of  mixing  greatly  changes  the  conditions 
of  precipitation. 

The  precipitate,  which  was  designated  Sample  R,  coagulated  and  settled 
quite  readily.    The  washing  and  drying  were  completed  as  usual. 

It  will  be  shown  that  samples  of  silver  phosphate  prepared  under  these  vari- 
ous conditions  have  nearly,  if  not  exactly,  the  same  composition.  Further 
proof  of  the  absence  of  acid  phosphate  in  these  samples  is  given  by  experiments 
to  be  described  later  which  show  that  no  water  is  given  off  when  this  material 
is  fused. 

An  attempt  to  prepare  a  sample  by  pouring  silver  nitrate  into  disodium  am- 
monium phosphate  yielded  imsatisfactory  results.  Since  the  disodium  am- 
monium phosphate  solution  was  alkaline,  owing  to  hydrolysis,  it  contained  free 
ammonia,  which  prevented  the  precipitation  of  silver  phosphate  at  first.  Nearly 
one  quarter  of  the  silver  nitrate  was  added  before  a  permanent  precipitate  was 
produced.  At  the  end  of  the  precipitation  the  solution  was  of  course  essentially 
neutral.  Even  after  standing  for  4  days  the  precipitate  had  not  appreciably 
settled.  Since  the  coagulation  of  the  precipitate  seems  to  occur  much  more 
readily  in  the  presence  of  excess  of  silver,  a  considerable  amount  of  silver 
nitrate  in  solution  was  added.  The  precipitate  coagulated  and  settled  immedi- 
ately. It  was  washed  and  dried  as  usual.  This  sample  was  somewhat  darker  in 
color  than  the  other  samples  and  gave  a  large  amount  of  insoluble  residue  when 
treated  with  dilute  nitric  acid.  The  analysis  showed  that  it  contained  about 
0.02  per  cent  too  much  silver.  This  method  of  preparation  is  evidently  unsatis- 
factory. 

Three  unsuccessful  attempts  were  made  to  prepare  silver  phosphate  from 
trisodium  phosphate.  The  samples  obtained  in  this  way  did  not  appear  homo- 
geneous after  being  dried  and  contained  considerable  sodiimi  in  spite  of  pro- 
tracted washing.  Two  of  these  samples  were  found  by  analysis  to  contain, 
respectively,  4.4  and  4.1  per  cent  less  silver  than  pure  trisilver  phosphate.  The 
third  of  these  samples  was  so  unsatisfactory  in  appearance  and  in  its  behavior 
during  its  preparation  that  it  was  not  analyzed.  This  method  of  preparing 
silver  phosphate  is  evidently  not  suitable  for  our  purpose.  Time  was  lacking 
to  investigate  further  this  anomalous  behavior. 


176  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

DRYING   OF   SILVER    PHOSPHATE. 

Unfortunately,  owing  to  the  high  melting-point  of  silver  phosphate,  it  was 
not  feasible  to  fuse  the  silver  phosphate  before  its  analysis  in  order  completely 
to  eliminate  all  water.  Instead  it  was  heated  in  a  platinum  boat,  in  a  current 
of  pure  dry  air,  at  a  temperature  of  about  400°  for  7  hours,  and  then  by  means 
of  the  botthng  apparatus  (page  8)  it  was  inclosed  in  its  weighing-bottle  with- 
out coming  in  contact  with  the  moist  air  of  the  laboratory.  During  this  heating 
the  access  of  light  to  the  sample  was  prevented.  The  continuous  current  of 
air  which  passed  over  the  silver  phosphate  during  the  heating  was  driven  by  a 
water  pump  successively  through  an  Emmerling  tower  containing  beads  mois- 
tened with  silver  nitrate  solution,  through  a  tower  containing  small  pieces  of 
fused  caustic  potash,  then  through  three  towers  containing  beads  drenched 
with  concentrated  sulphuric  acid,  and  finally  through  a  long  tube  containing 
phosphorus  pentoxide  which  had  been  resublimed  in  a  current  of  air.  The 
hard-glass  tube  containing  the  platinum  boat  was  surrounded  by  blocks  of 
aluminum  (page  78)  which  were  jacketed  with  asbestos  on  the  top  and  sides 
and  heated  directly  from  below  by  a  large  burner.  The  platinum  boat  was  not 
attacked  in  the  least,  as  was  shown  by  the  fact  that  its  weight  remained  con- 
stant. 

It  was  feared  that  in  spite  of  this  prolonged  heating  the  silver  phosphate  still 
retained  a  trace  of  water,  but  by  making  the  conditions  in  the  different  experi- 
ments as  nearly  uniform  as  possible  it  was  hoped  that  the  amount  of  water 
retained  would  be  constant.  Proof  will  be  given  later  that  the  drying  was 
highly  efficient. 

The  salt  thus  prepared  for  analysis  was  allowed  to  stand  over  night  in  a 
desiccator  covered  with  a  black  cloth  in  the  balance  room,  and  was  then  weighed 
in  its  glass-stoppered  bottle  by  substitution,  with  the  use  of  another  weighing- 
bottle  of  very  similar  surface  and  volume  as  a  counterpoise. 

The  balance  was  a  nearly  new  No.  10  Troemner  balance.  It  was  easily  sen- 
sitive to  0.02  mg.  The  weights  had  already  been  used  in  an  investigation  of 
the  atomic  weight  of  sulphur,^  and  were  restandardized  with  a  very  gratifying 
result.  None  of  the  corrections  found  differed  by  as  much  as  0.02  mg.  from  those 
found  a  year  before,  and  only  a  few  by  o.oi  mg.  The  balance  was  provided  with 
a  few  miUigrams  of  radium  bromide  of  radio-activity  loooo  to  dispel  electri- 
cal charges  generated  during  the  handling  of  the  weighing-bottles  with  cork- 
tipped  pincers. 

1  Richards  and  Jones:  Pub.  Car.  Inst.,  No.  69,  69  (1907);  Jour.  Amer.  Chem.  Soc,  29,  826; 
Zeit.  anorg.  Chetn.,  55,  72. 


A  REVISION   OF    THE   ATOMIC   WEIGHT    OF   PHOSPHORUS.  177 

THE   DETERMINATION    OF   SILVER  IN   SILVER   PHOSPHATE. 

The  platinum  boat  containing  the  silver  phosphate  was  transferred  to  an 
Erlenmeyer  flask  of  "non-sol"  glass  of  i  Hter  capacity  and  treated  with  about 
30  c.c.  of  5  normal  nitric  acid.  Solution  took  place  rapidly.  The  solution  was . 
not  perfectly  clear,  however,  owing  to  a  very  slight  insoluble  residue  which 
sometimes  settled  out  on  standing.  The  solution  was  then  heated  on  a  steam 
bath  until  the  residue  dissolved  completely.  Upon  the  addition  of  about  one 
hter  of  cold  water  a  very  slight  opalescence  was  produced,  which  was  visible 
only  when  the  solution  was  carefully  examined  in  a  very  favorable  Hght.  The 
solution  was  again  warmed  imtil  it  became  perfectly  clear.  The  water  and 
nitric  acid  used  in  these  processes  did  not  give  an  opalescence  visible  in  the 
nephelometer  when  treated  with  silver  nitrate.  The  nature  of  this  residue  will 
be  discussed  more  in  detail  after  describing  the  remainder  of  the  analytical 
process. 

About  800  c.c.  of  water  was  placed  in  a  large  glass-stoppered  precipitating 
flask  and  a  very  slight  excess  of  hydrobromic  acid  was  added  from  a  burette. 
The  silver  phosphate  solution  was  then  very  carefully  poured  into  the  hydro- 
bromic acid  solution.  This  method  of  precipitation  gives  less  opportunity  for 
the  occlusion  of  silver  phosphate  or  nitrate  than  the  reverse  method.  The  oc- 
clusion of  hydrobromic  acid  can  do  no  harm.  The  flask  was  shaken  for  20 
minutes  and  was  allowed  to  stand  for  several  days  until  the  precipitate  had 
completely  settled.  Then  the  precipitate  was  coUected  upon  a  weighed  Gooch 
crucible  after  many  rinsings  with  pure  water.  In  order  to  protect  the  mat  of 
the  Gooch  crucible  from  disintegration,  it  was  covered  by  a  circular  disk  of  thin 
platinimi  foil,  perforated  with  many  small  holes.  The  precipitate  was  dried  in 
an  electrically  heated  air  bath  for  several  hours  at  90°,  then  for  some  time  at 
130°,  and  finally  for  at  least  eight  hours  at  180°.  After  the  crucible  containing 
the  precipitate  had  been  weighed,  the  silver  bromide  was  transferred  to  a  por- 
celain crucible  and  the  loss  on  fusion  determined. 

The  presence  of  the  platinum  disk  covering  the  mat  makes  it  possible  to  trans- 
fer very  nearly  all  the  silver  bromide  to  the  porcelain  crucible  without  contam- 
ination with  asbestos  and  therefore  it  is  unnecessary  to  correct  the  loss  on 
fusion  for  the  small  amount  of  silver  bromide  which  is  not  fused.  The  loss  on 
fusion,  which  represents  water  remaining  in  the  silver  bromide,  was  subtracted 
from  the  weight  of  the  silver  bromide.  The  asbestos  shreds  carried  away  by 
the  wash-waters  and  any  silver  bromide  which  may  have  escaped  the  Gooch 
crucible  were  collected  by  passing  the  filtrate  through  a  very  small  filter  paper. 
The  paper  was  then  burned  and  the  residue,  after  treatment  with  a  drop  of 
nitric  and  hydrobromic  acids  to  convert  any  reduced  silver  into  silver  bromide, 
was  again  gently  heated  and  finally  was  weighed.  The  weight  of  the  asbestos, 
corrected  for  the  ash  of  the  paper,  was  added  to  the  weight  of  the  silver 
bromide. 


178  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

In  order  to  determine  the  soluble  silver  bromide,  the  filtrate  was  evaporated 
until  m.ost  of  the  excess  of  nitric  acid  was  driven  off.  The  precipitating  flask 
and  all  the  flasks  which  had  held  the  filtrate  were  rinsed  with  strong  ammonia 
and  the  rinsings  added  to  the  evaporated  wash-water.  Enough  ammonia  was 
added  to  make  the  solution  alkaline  and  it  was  then  diluted  to  100  c.c.  in  a 
graduated  flask.  The  amount  of  silver  bromide  present  was  determined  by 
comparison  in  the  nephelometer  with  a  very  similar  solution  containing  a 
known  amount  of  silver  bromide.  Both  precipitates  were  dissolved  in  ammonia 
and  reprecipitated  at  the  same  time  and  under  precisely  similar  conditions  in 
the  nephelomxCter  tubes  by  a  slight  excess  of  nitric  acid.  The  amount  found  in 
this  way  was  added  to  the  weight  of  the  silver  bromide. 

In  order  to  determine  whether  silver  phosphate  is  occluded  by  silver  chloride, 
about  6  gm.  of  silver  phosphate  were  dissolved  in  nitric  acid  and  the  solution 
was  diluted  and  poured  into  an  excess  of  hydrochloric  acid.  After  standing  until 
the  supernatant  liquid  was  clear,  the  precipitate  was  washed  very  thoroughly 
with  water  and  then  dissolved  in  redistilled  ammonia.  The  solution  was  diluted 
to  I  liter  and  the  silver  chloride  was  reprecipitated  with  nitric  acid.  The  precipi- 
tate was  filtered  out  and  the  filtrate  evaporated  in  a  platinum  dish  until  concen- 
trated. A  little  sodium  carbonate  was  added  and  the  dish  was  heated  to  expel 
all  volatile  ammonium  salts.  The  residue  was  dissolved  in  about  3  c.c.  of  water, 
and  treated  with  an  excess  of  ammonium  molybdate  reagent  with  gentle  warm- 
ing. After  standing  for  3  days',  not  the  slightest  precipitate  dr  yellow  color  had 
appeared,  showing  that  no  phosphate  had  been  occluded  by  the  silver  chloride. 
Although  not  tested  experimentally,  it  is  reasonable  to  suppose  that  silver 
bromide  also  does  not  possess  the  property  of  occluding  appreciable  quantities 
of  silver  phosphate  or  phosphoric  acid. 

INSOLUBLE   RESIDUE. 

The  presence  of  a  slight  residue  or  opalescence,  after  dissolving  the  dried 
silver  phosphate  in  dilute  nitric  acid,  proved  the  most  perplexing  difficulty 
which  was  encountered.  The  effort  to  discover  the  nature  of  this  insoluble 
matter  and  eliminate  it  consumed  a  large  part  of  the  time  devoted  to  this 
research.  In  an  effort  to  make  sure  that  it  was  not  due  to  some  unknown 
im-purity,  nineteen  different  samples  of  silver  phosphate  were  prepared,  the 
source  of  material,  method  of  purification,  and  method  of  precipitation  being 
varied.  Disodium  phosphate,  trisodium  phosphate,  and  sodium  ammonium 
phosphate  were  carefully  purified  and  converted  into  silver  phosphate  under 
varying  conditions  without  appreciable  effect  upon  the  amount  of  the  residue. 
Phosphorus  oxychloride  was  twice  fractionally  distilled,  converted  into  phos- 
phoric acid,  and  then  into  disodium  phosphate  by  means  of  sodium  hydroxide 
made  from  sodium  amalgam.  The  product  was  crystallized  three  times.  Silver 
phosphate  made  from  this  material  gave  a  slight  residue,  very  similar  to  that 
obtained  from  the  best  samples  made  in  other  ways.    Unfortunately,  it  was 


A  REVISION  OF  THE   ATOMIC  WEIGHT   OF  PHOSPHORUS.  179 

necessary  to  reject  the  analytical  results  obtained  with  this  specimen  because  it 
was  found  to  contain  a  small  amount  of  metaphosphate.  We  did  not  succeed 
in  preparing  a  sample  of  silver  phosphate  entirely  free  from  the  residue. 

In  the  meantime  attention  had  been  devoted  to  the  residue  itself.  The  small 
amount  of  material  available  rendered  this  part  of  the  investigation  difficult. 
The  silver  phosphate,  after  precipitation  and  washing,  but  undried,  dissolves 
in  dilute  nitric  acid,  giving  a  solution  which  is  perfectly  clear  to  the  naked  eye, 
although  some  samples  gave  a  barely  visible  opalescence  in  the  nephelometer. 
The  opalescence  was  much  too  small  to  have  any  effect  on  the  analytical  results. 
The  dried  samples  invariably  gave  an  opalescence. 

Dry  silver  phosphate  is  very  slowly  darkened  in  color  by  the  action  of  light. 
This  effect  is  even  more  pronounced  when  silver  phosphate  is  exposed  to  the  light 
in  the  presence  of  water.  These  darkened  samples  gave  a  much  greater  residue 
than  the  undarkened  material.  The  residue  was  insoluble  in  ammonia,  slowly 
soluble  in  dilute  nitric  acid,  especially  when  heated,  and  readily  soluble  in  strong 
nitric  acid.  The  addition  of  hydrochloric  acid  to  these  nitric  acid  solutions  gave 
a  precipitate  of  silver  chloride,  while  ammonium  molybdate  indicated  the  pres- 
ence of  phosphate. 

In  order  to  determine  whether  or  not  a  loss  of  weight  occurs  during  the 
darkening  by  light,  a  sample  of  silver  phosphate  was  dried  and  weighed  as 
usual  and  found  to  weigh  3.01901  gm.  It  was  then  exposed  to  the  direct  action 
of  bright  sunhght  for  a  day,  while  contained  in  a  weighing-bottle  which  was 
placed  in  a  desiccator  over  sulphuric  acid.  It  was  foimd  to  have  darkened 
slightly  in  color  and  to  weigh  3.01903.  The  gain  of  0.02  mg.  is  within  the  limit 
of  error  in  the  weighing.  This  sample,  when  treated  with  dilute  nitric  acid, 
gave  a  much  larger  residue  than  usual,  which  weighed  1.8  mg.  This  is  much  more 
residue  than  was  usually  found  in  samples  containing  from  4  to  8  gm.  of  silver 
phosphate.  It  is  estimated  that  the  samples  which  had  been  protected  from  the 
action  of  light  as  much  as  possible,  except  when  unavoidably  exposed  to  diffused 
dayhght  while  being  weighed  or  transferred  to  the  furnace  and  solution  flask, 
contained  about  o.oi  per  cent  of  this  residue. 

Two  analyses  were  made  of  the  residue  obtained  by  exposing  silver  phosphate 
under  water  to  the  action  of  light  for  several  days,  then  dissolving  the  excess  of 
silver  phosphate  in  dilute  nitric  acid  and  thoroughly  washing  and  drpng  the 
residue.  0.02674  gm.  of  this  residue  yielded  0.03551  gm.  of  silver  chloride,  which 
indicates  that  the  residue  contained  99.9  per  cent  of  silver.  In  the  case  of  an- 
other sample  of  the  residue  prepared  and  analyzed  in  the  same  way,  0.04320 
gm.  of  residue  yielded  0.05747  gm.  of  silver  chloride,  which  indicates  that  the 
residue  contained  100.  i  per  cent  of  silver.  The  mean  of  the  two  analyses  is  loo.o 
per  cent  of  silver.  These  analyses  prove  conclusively  that  when  silver  phosphate 
is  acted  on  by  Hght  in  the  presence  of  water,  it  is  so  altered  (perhaps  by  the  for- 
mation of  a  subphosphate  similar  to  subchloride),  that  when  treated  with  very 
dilute  nitric  acid  metallic  silver  remains. 


l8o  RESEARCHES   UPON   ATOMIC   WEIGHTS. 

It  does  not  follow,  however,  that  it  would  be  a  correct  procedure  to  deter- 
mine the  per  cent  of  this  residue  obtained  from  the  samples  used  for  analysis  and 
apply  a  correction  on  the  assumption  that  the  material  consisted  of  pure  silver 
phosphate  and  a  small  amount  of  pure  silver.  This  procedure  would  assume 
that  the  other  product  of  decomposition  is  eliminated  and  not  weighed.  There 
are  two  facts  which  show  that  this  assumption  would  be  incorrect.  In  nearly 
every  analysis,  when  the  solution  was  diluted,  after  bringing  the  residue  into 
solution  by  heating  on  the  steam  bath,  a  slight  opalescence  was  produced. 
Careful  tests  of  the  water  used  showed  that  this  opalescence  was  not  due  to  im- 
purity in  the  water.  It  seems  probable  that  the  substance  which  caused  this 
opalescence  was  derived  in  part  from  the  phosphate  radical  during  the  decom- 
position which  produced  the  residue.  The  other  fact  is  that  dry  silver  phosphate 
does  not  lose  weight  when  darkened  by  exposure  to  sunlight,  although  this 
treatment  increases  the  amount  of  residue. 

The  conclusion  in  regard  to  this  residue  may  be  summarized  as  follows: 
The  washed  moist  silver  phosphate  was  free  from  residue  and  contained  silver 
and  phosphoric  acid  combined  in  atomic  proportions.  During  the  drying  and 
weighing  a  slight  decomposition  took  place,  undoubtedly  owing  in  part  at  least 
to  the  action  of  light.  It  seems  probable  that  during  this  decomposition  no 
loss  in  weight  took  place,  and  therefore  the  sample  contained  the  proper  per- 
centage of  silver.  When  this  slightly  darkened  silver  phosphate  is  treated  with 
cold  dilute  nitric  acid,  the  unchanged  silver  phosphate  and  perhaps  also  a  por- 
tion of  the  altered  material  dissolve,  leaving  a  slight  opalescence,  which  in 
some  cases  is  deposited  as  a  very  slight  residue  on  standing.  This  residue  is 
estimated  to  be  about  o.oi  per  cent  of  the  weight  of  the  silver  phosphate.  When 
the  solution  is  warmed  until  perfectly  clear,  and  then  diluted,  a  very  slight 
opalescence  is  usually  produced  which  could  be  again  cleared  up  by  warming  the 
solution.  This  opalescence  is  probably  caused  by  the  presence  of  the  altered 
phosphate  anion.  If  this  explanation  is  correct,  the  presence  of  the  residue 
can  not  influence  the  result,  and  no  correction  need  be  appHed.  Until  the  exact 
nature  of  the  decomposition  products  can  be  determined,  there  must  remain 
some  uncertainty  in  regard  to  whether  or  not  any  correction  is  necessary. 

The  uncertainty  from  this  cause  is,  however,  not  very  great.  Even  if  all  the 
phosphorus  and  oxygen  corresponding  to  the  residue  of  silver  is  removed  before 
the  weighing,  the  correction  would  be  only  23  per  cent  of  the  weight  of  the 
residue.  If  the  residue  amounts  to  o.oi  per  cent,  as  has  been  estimated,  the 
maximum  correction  would  be  0.002  per  cent.  If  part  of  the  oxygen  is  lost,  but 
the  phosphorus  remains,  the  correction  would  of  course  be  smaller.  If  there  is 
no  loss  in  weight  by  the  action  of  Hght  on  the  dry  silver  phosphate,  no  correc- 
tion need  be  applied.  From  the  evidence  so  far  obtained  the  latter  assumption 
seems  rather  more  probable  than  any  of  tiie  others,  and  therefore  no  correction 
has  been  applied. 


A  REVISION   OF  THE   ATOMIC  WEIGHT   OF  PHOSPHORUS. 


i8i 


DETERMINATION    OF    MOISTQRE   IN    THE    DRIED    SILVER 

PHOSPHATE. 

In  order  to  find  out  how  efficient  the  drying  of  the  silver  phosphate  had  been, 
experiments  were  made  to  determine  the  amount  of  water  retained  by  silver 
phosphate  which  had  been  dried  for  analysis  as  described  above.  (See  page 
176.)  The  water  was  determined  by  fusing  the  dried  phosphate  in  a  current 
of  dry  air  and  collecting  the  moisture  set  free  in  a  weighed  phosphorus  pentoxide 
tube.  Since  the  melting  point  of  pure  silver  phosphate  is  considerably  above 
the  softening  point  of  hard  glass,  it  was  found  advantageous  to  lower  the  melt- 
ing point  of  the  phosphate  by  the  use  of  silver  chloride  as  a  flux. 

About  15  gm.  of  silver  phosphate  were  placed  in  one  end  of  a  large  silver 
boat  and  in  the  other  end  about  12  gm.  of  previously  fused  silver  chloride. 
The  boat  was  then  inserted  in  a  hard-glass  tube  and  dried  under  the  same  con- 
ditions as  prevailed  in  preparing  the  samples  for  the  determination  of  the  silver 
content.  After  the  silver  phosphate  had  been  heated  for  seven  hours  in  a  current 
of  purified  air  dried  by  phosphorus  pentoxide,  the  air  passing  over  the  boat  in 
the  furnace  was  conducted  through  a  weighed  U-tube  containing  resublimed 
phosphorus  pentoxide  for  30  minutes.  This  was  done  to  make  sure  that  all  the 
water  which  had  been  hberated  from  the  silver  phosphate  without  fusion  had 
been  swept  out  of  the  apparatus.  In  no  case  was  there  a  gain  in  weight  during 
this  process  of  more  than  0.05  mg.,  which  is  about  the  limit  of  error  in  weighing 
the  phosphorus  pentoxide  tubes.  The  backward  diffusion  of  moisture  was  pre- 
vented by  a  second  tube  containing  pentoxide. 

The  carefully  weighed  phosphorus  pentoxide  tube  was  again  attached  to  the 
tube  containing  the  silver  boat  with  its  charge  of  silver  phosphate  and  silver 
chloride.  The  latter  tube  was  then  heated  hot  enough  to  fuse  the  silver  chlo- 
ride, which  flowed  down  to  the  silver  phosphate  and  readily  caused  the  entire 
charge  to  fuse  completely.  The  liberated  water  was  swept  into  the  phosphorus 
pentoxide  tube  by  a  current  of  dry  air  for  about  30  minutes.  The  tube  was 
then  reweighed  to  determine  the  water  evolved  by  the  fusion  of  silver  phosphate. 
The  pentoxide  tube  was  weighed  by  substitution  for  a  very  similar  counter- 
poise tube,  one  stopcock  of  each  tube  being  open  during  the  weighing.  Before 
being  weighed  both  tubes  were  wiped  with  a  damp  cloth  and  allowed  to  stand 
near  the  balance  for  at  least  30  minutes. 

The  following  table  gives  the  results  of  these  experiments: 


Sample. 

Weight  of  silver 
phosphate. 

Weight  of 
water. 

Per  cent 
of  water. 

P 
P 
0 
0 

13-50 
15.64 
15-66 
16.62 

0.00012 
0.00007 
0.00005 
0.00003 

0.0009 
0.0004 
0.0003 
0.0002 

A 

verage      0.0005 

l82 


RESEARCHES  UPON  ATOMIC  WEIGHTS. 


The  amount  of  water  evolved  is  hardly  greater  than  the  probable  error  in 
weighing  the  phosphorus  pentoxide  tubes,  and  is  less  than  the  probable  error 
in  determining  the  amount  of  silver  in  the  salt.  We  are  therefore  justified  in 
concluding  that  the  material  which  was  used  for  the  determination  of  silver  was 
essentially  free  from  water  and  that  no  correction  need  be  appUed  to  the  results 
for  inefi&cient  drying. 

This  result  also  furnishes  evidence  that  the  samples  are  free  from  acid  phos- 
phates, which,  owing  to  conversion  into  pyrophosphate  or  metaphosphate,  would 
evolve  water  when  fused,  although  it  is  possible  that  occluded  acid  phosphates 
might  have  been  converted  into  pyrophosphate  or  metaphosphate  during  the 
drying.  Sample  O,  which  was  prepared  under  conditions  most  favorable  for  the 
formation  of  the  acid  silver  phosphate,  does  not  appear  to  contain  more  water 
than  Sample  P,  which  was  prepared  under  conditions  which  were  unfavorable 
to  the  formation  of  acid  phosphate.  Since  these  two  samples,  which  differed 
most  widely  in  their  method  of  preparation,  showed  no  difference  in  the  amount 
of  water  retained,  it  seemed  unnecessary  to  test  the  other  samples  also.  Unfor- 
tunately this  method  of  detecting  acid  phosphate  is  not  very  sensitive,  owing 
to  the  unfavorable  relation  of  the  atomic  weights  involved  —  one  molecule  of 
water  corresponding  to  a  deficiency  of  two  atoms  of  silver. 


THE   SPECIFIC   GRAVITY    OF    SILVER   PHOSPHATE. 

In  order  that  the  apparent  weight  of  the  silver  phosphate  might  be  corrected 
to  the  vacuima  standard,  the  specific  gravity  of  this  salt  was  found  by  deter- 
mining the  weight  of  toluol  displaced  by  a  known  quantity  of  salt.  The  spe- 
cific gravity  of  the  toluol  at  25°  referred  to  water  at  4°  was  0.8633.  Great  care 
was  taken  to  remove  air  from  the  salt  when  covered  with  the  toluol  by  warming 
the  pycnometer,  then  placing  it  in  a  vacuiun  desiccator  and  boiHng  the  toluol 
under  reduced  pressure.  The  salt  and  toluol  were  mechanically  stirred  to  assist 
the  escape  of  air  bubbles.    This  process  was  repeated  several  times. 


The  SPEcrFic  GRAvmi 

OF  Silver  Phosphate. 

Weight  of 

silver  phosphate 

in  vacuum. 

Weight  of 

displaced  toluol 

in  vacuum. 

Volume  of 
silver  phosphate. 

Density  of 
silver  phosphate. 

gm. 
22.955 
16.942 

gm. 

3.I13 
2.29s 

e.c. 
3.606 
2.658 

6.366 
6.374 

Mean 6.37 

Therefore  the  apparent  weight  of  silver  phosphate  was  corrected  to  the  vacuum 
standard  by  adding  0.000044  gm.  per  gram  of  salt.  Similarly  0.000041  gm. 
was  added  for  every  gram  of  silver  bromide. 


A  REVISION  OF  THE  ATOMIC  WEIGHT  OF  PHOSPHORUS.  183 

ADSORPTION   OF  AIR   BY   SILVER   PHOSPHATE. 

Since  the  silver  phosphate  was  in  a  very  JSnely  divided  condition  and  since 
many  fine  powders  have  the  power  of  adsorbing  appreciable  quantities  of  air  or 
other  gases,  the  possibility  of  the  adsorption  of  air  by  silver  phosphate  was  in- 
vestigated. The  method  of  experimenting  and  the  apparatus  were  very  similar 
to  that  used  by  Baxter  and  Tilley  for  investigating  the  behavior  of  iodine 
pentoxide: 

Two  weighing-bottles  were  constructed  with  long,  very  well  ground  stoppers  which  ter- 
minated in  stopcocks  through  which  the  tubes  could  be  exhausted.  These  tubes  were  very 
closely  of  the  same  weight  and  of  very  nearly  the  same  internal  capacity.  The  tubes  were  first 
exhausted  and  compared  in  weight  by  substitution.  Next  they  were  filled  with  dry  air  and 
again  weighed,  the  weighing  being  carried  out  with  stopcocks  open.  Both  steps  were  then 
repeated  with  essentially  the  same  results.^ 

In  these  two  experiments,  when  air  was  admitted,  the  counterpoise  gained 
0.00028  and  0.0002 1  gm.  respectively  (average  0.00025)  more  than  the  tube  which 
was  later  to  contain  the  silver  phosphate.  After  22.69  gm.  of  pure  dry  silver 
phosphate  had  been  placed  in  the  tube,  the  tube  and  its  counterpoise  were  ex- 
hausted and  the  difference  in  weight  determined.  When  dry  air  at  25°  C.  and 
766  mm.  was  admitted  to  both  the  tube  containing  the  silver  phosphate  and  the 
counterpoise,  the  counterpoise  gained  0.00443  &^-  more  than  the  tube.  There- 
fore, the  air  displaced  by  the  silver  phosphate  was  0.00443  —  0.00025  = 
0.00418  gm.  Since  22.69  g"^-  of  silver  phosphate  of  density  6.37  have  a  volume 
of  3.56  c.c,  the  volume  of  pure  air  displaced  at  25°  C.  and  766  mm.  should 
weigh  0.00425  gm.2 

The  experiment  was  then  repeated.  After  the  air  had  been  exhausted  from  the 
tube  and  its  counterpoise,  the  tube  containing  the  silver  phosphate  was  heated 
gently.  No  gas  was  evolved.  The  tube  and  its  counterpoise  were  then  weighed 
by  substitution.  When  dry  air  at  24.5°  and  767  mm.  was  admitted  to  both,  the 
counterpoise  gained  0.00445  gm.  more  than  the  tube  containing  the  silver 
phosphate.  Therefore  the  air  displaced  by  the  silver  phosphate  was  0.00445  — 
0.00025  =  0.00420  gm.,  whereas  the  weight  of  air  displaced,  calculated  from  the 
density  of  the  salt,  is  0.00426  gm. 

The  agreement  between  the  experimental  results  and  those  calculated  from 
the  denstiy  of  silver  phosphate  on  the  assimiption  that  no  adsorption  takes 
place  is  close  enough  to  show  that  no  significant  amount  of  adsorption  occurs. 

1  Baxter  and  Tilley:  Jour.  Amer.  Chem.  Soc,  31,  214  (1909);  Zeit.  anorg.  Chetn.,  61,  310. 
(See  page  130.) 

^  Rayleigh's  value  for  the  density  of  air  at  0°  and  760  mm.,  1.293  g°i-  P^r  ^**®^>  ^^  used. 
Proc.  Roy.  Soc,  53,  147. 


i84 


RESEARCHES    UPON   ATOMIC   WEIGHTS. 


RATIO    OF    SILVER    BROMIDE    TO    SILVER    PHOSPHATE. 

The  following  table  contains  all  of  the  analyses  not  vitiated  by  a  known  im- 
purity in  the  sample  or  by  an  accident  during  the  analysis.  One  feature  of  this 
table  requires  further  explanation.  In  analysis  5  the  silver  was  determined  by 
precipitation  as  chloride  instead  of  bromide.  For  every  gram  of  silver  phos- 
phate there  was  obtained  1.02727  gm.  of  silver  chloride.  Since  Baxter  found 
AgBr:AgCl=  1,31017:1.00000,  this  analysis  indicates  that  one  gram  of 
sample  N  is  equivalent  to  1.02704  X  1.31017  =  1.34560  gm.  of  silver  bromide. 
This  result  is  placed  in  the  table  for  comparison  with  the  other  analyses  and  is 
used  in  the  computation  of  the  mean. 

Series  I.    3  AgBr:  AgsPO^. 


No.  of 

Sample 

Weight  of 

Weight  of 

Weight 

Dissolved 
AgBr. 

Loss 

Corrected 

Ratio 

analy- 

of 

AgsPO* 

AgBr  in 

of 

on 

weight  of 

3AgBr 

sis. 

AgsPOi 

m  vacuum. 

vacuum. 

asbestos. 

fusion. 

of  AgBr. 

AgsPOj. 

gm. 

gm. 

gm 

gm. 

gm. 

gm. 

I 

0 

6.20166 

8.34427 

0.00036 

0.00034 

0.00007 

8.34490 

1-34558 

2 

0 

6.35722 

8.55386 

0.00041 

0.00003 

0. 0001 1 

8.55419 

I-345S9 

3 

N 

5.80244 

7.80792 

0.00029 

0.00005 

0.00007 

7.80819 

1-34567 

4 

N 

5-05845 

6.80658 
(AgCl) 

0.00019 

0.00020 

0.00012 

6.80685 
/     AgCl     \ 
\  3-43544  / 

1-34564 

5 

N 

3-34498 

3-43514 

0.00029 

0.00009 

0.00008 

1.34560 

6 

P 

7-15386 

9.62648 

0.00046 

0.00013 

0.00013 

9.62694 

1-34570 

7 

P 

7.20085 

9.68929 

0.00023 

0.00005 

O.OOOIO 

9.68947 

1.34560 

8 

R 

6.20182 

8.34466 

0.00041 

0.00027 

0.00012 

8.34522 

1-34561 

9 

R 

5.20683 

7-00543 

0.00029 

0.00040 

0.00007 

7.00605 

1-34555 

Ave  rag 

e     .    .    .    . 

1.34562 

Per  cent 

of  Ag  in  A 

K.PO.    .    . 

.      77.300 

DISCUSSION    OF    RESULTS. 

A  careful  study  of  these  results  shows  that  the  composition  of  silver  phosphate 
is  very  nearly,  if  not  quite,  independent  of  the  changes  in  the  acidity  of  the  solu- 
tions from  which  it  is  precipitated.  Samples  O  and  R  were  prepared  under 
slightly  more  acid  conditions  than  Samples  N  and  P.  The  average  amount  of 
silver  bromide  obtained  from  one  gram  of  Samples  0  and  R  is  1.34558  (77.297 
per  cent  of  silver),  whereas  the  average  from  Samples  N  and  P  is  1.34564 
(77.301  percent  of  silver).  This  difference,  if  real  and  significant,  is  probably 
due  to  a  very  slight  occlusion  of  disilver  hydrogen  phosphate.  It  does  not  seem 
probable  that  any  basic  salt  was  present  in  Samples  N  and  P,  because  silver 
shows  little  tendency  to  form  basic  salts  and  the  conditions  of  precipitation 
were  not  favorable  for  the  formation  of  basic  salts. 

The  difference  between  composition  of  the  samples  is  so  slight,  both  in  abso- 
lute amount  and  by  comparison  with  the  differences  between  different  analyses 
of  the  same  sample,  that  in  the  present  state  of  our  knowledge  it  does  not 

^  Loc.  cit. 


A  REVISION   OF   THE   ATOMIC  WEIGHT   OF  PHOSPHORUS.  185 

seem  justifiable  to  reject  the  analyses  of  Samples  R  and  0.  This  conclusion  is 
supported  by  the  fact  that  the  water  determinations  failed  to  show  a  difference 
between  these  samples.  The  results,  however,  indicate  that  the  average  ratio 
1.34562  (77.300  per  cent  of  silver)  may  be  very  slightly  too  low,  owing  to  the 
presence  of  disilver  hydrogen  phosphate.  The  ratio  1.34562,  assuming  the 
atomic  weight  of  silver  to  be  107.880,  and  assimiing  that  silver  bromide  con- 
tains 57.4453  per  cent  of  silver,  leads  to  an  atomic  weight  of  31.043  for  phos- 
phorus, whereas  the  ratio  1.34564  derived  from  Samples  N  and  P  gives  the  value 
31.037.  The  rounded-off  value,  31.04,  may  be  considered  to  be  essentially  free 
from  error  from  this  source. 

SUMMARY. 

1.  A  careful  study  has  been  made  of  the  conditions  necessary  for  the  prep- 
aration of  pure  trisilver  phosphate. 

2.  It  is  found  that  silver  phosphate  can  be  almost  completely  dried  without 
fusion  by  heating  in  a  current  of  dry  air. 

3.  The  density  of  silver  phosphate  is  foimd  to  be  6.37. 

4.  It  is  found  that  silver  phosphate  does  not  adsorb  a  significant  amou,nt 
of  air. 

5.  Nine  analyses,  made  with  four  different  s'amples,  show  that  one  gram  of 
silver  phosphate  yields  1.34562  gm.  of  silver  bromide,  whence  the  per  cent  of 
silver  in  silver  phosphate  is  77.300. 

Therefore, 

If  Ag  =  107.88  P  =  31.04 

If  Ag=  107.87  P  =  31-03 


Date  Due 

nnj  n  bi 

984 

1 

IJAV      1   i 

MAY    1  ^ 

i999 

' 

I 

f) 

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